diff --git a/app/components/.client/ImageViewer/components/canvas/Overlays/OverlaysLayer.tsx b/app/components/.client/ImageViewer/components/canvas/Overlays/OverlaysLayer.tsx
index 37b49c82..52f96ca9 100644
--- a/app/components/.client/ImageViewer/components/canvas/Overlays/OverlaysLayer.tsx
+++ b/app/components/.client/ImageViewer/components/canvas/Overlays/OverlaysLayer.tsx
@@ -3,6 +3,7 @@ import { type _TileLoadProps as TileLoadProps, TileLayer } from "@deck.gl/geo-la
 import { PolygonLayer } from "@deck.gl/layers";
 import { type Table } from "apache-arrow";
 
+import { additiveBlendParameters } from "./additiveBlending.glsl";
 import { AdditivePolygonLayer } from "./AdditivePolygonLayer";
 import { AdditiveScatterplotLayer } from "./AdditiveScatterplotLayer";
 import { getPolygon } from "./getPolygon";
@@ -192,8 +193,11 @@ export const OverlaysLayer = ({
           },
         };
 
+        // Explicit: the TileLayer's defaulted `parameters: {}` would clobber a
+        // subclass defaultProps value through the `...props` spread.
         return new AdditiveScatterplotLayer({
           ...props,
+          parameters: additiveBlendParameters,
           data,
           getRadius: pointRadius,
           radiusMinPixels: pointRadiusMin,
@@ -254,6 +258,7 @@ export const OverlaysLayer = ({
       const fillLayer = new AdditivePolygonLayer({
         ...props,
         id: `${props.id}-fill`,
+        parameters: additiveBlendParameters,
         data: arrowTable,
         getPolygon: (_d: unknown, context: AccessorContext<unknown>) =>
           polygonAccessor(context.index, context),
diff --git a/app/components/.client/ImageViewer/components/canvas/Overlays/__tests__/additiveBlending.test.ts b/app/components/.client/ImageViewer/components/canvas/Overlays/__tests__/additiveBlending.test.ts
new file mode 100644
index 00000000..76e9111d
--- /dev/null
+++ b/app/components/.client/ImageViewer/components/canvas/Overlays/__tests__/additiveBlending.test.ts
@@ -0,0 +1,114 @@
+import { vi } from "vitest";
+
+import type { CellMarker } from "../../../../state/store/types";
+import { additiveBlendParameters } from "../additiveBlending.glsl";
+import { createMarkerProps } from "../markerUniforms";
+import { OverlaysLayer } from "../OverlaysLayer";
+
+// deck.gl's TileLayer constructor pulls in the whole runtime (luma.gl, gl
+// matrices, etc.) which happy-dom can't handle. We only care about the
+// props object the layer was constructed with, so stub TileLayer to a
+// minimal class that captures the props verbatim.
+vi.mock("@deck.gl/geo-layers", () => ({
+  TileLayer: class {
+    props: Record<string, unknown>;
+    constructor(props: Record<string, unknown>) {
+      this.props = props;
+    }
+  },
+}));
+
+vi.mock("@deck.gl/layers", () => ({
+  PolygonLayer: class {
+    constructor(public props: Record<string, unknown>) {}
+  },
+}));
+
+vi.mock("../AdditiveScatterplotLayer", () => ({
+  AdditiveScatterplotLayer: class {
+    constructor(public props: Record<string, unknown>) {}
+  },
+}));
+
+vi.mock("../AdditivePolygonLayer", () => ({
+  AdditivePolygonLayer: class {
+    constructor(public props: Record<string, unknown>) {}
+  },
+}));
+
+vi.mock("~/toast-bridge", () => ({
+  toastBridge: { emit: vi.fn() },
+}));
+
+const makeFileMarkers = (): Record<string, CellMarker> => ({
+  marker_positive_CD3: {
+    color: [255, 0, 0, 1],
+    count: 0,
+    isVisible: true,
+  },
+});
+
+const buildLayer = () => {
+  const fileMarkers = makeFileMarkers();
+  const layer = OverlaysLayer({
+    resourceId: "res-1",
+    overlayConfig: null,
+    fileMarkers,
+    enabledMarkers: ["marker_positive_CD3"],
+    markerProps: createMarkerProps(fileMarkers, 0.5),
+    imageWidth: 1024,
+    imageHeight: 1024,
+    minZoom: 0,
+    maxZoom: 8,
+    strokeOpacity: 1,
+    loadTile: vi.fn(),
+    finishTile: vi.fn(),
+  });
+
+  return layer as unknown as {
+    props: { renderSubLayers: (props: Record<string, unknown>) => unknown };
+  };
+};
+
+const emptyArrowTable = { numRows: 0, getChild: () => ({ data: [] }) };
+
+describe("OverlaysLayer additive blending", () => {
+  test("fill sublayers receive the additive blend parameters (parent props must not clobber them)", () => {
+    const layer = buildLayer();
+
+    // Polygon mode (z >= -2): [fill, stroke].
+    const [fill] = layer.props.renderSubLayers({
+      id: "MarkersLayer-res-1-tile",
+      data: emptyArrowTable,
+      tile: { index: { z: 0, x: 0, y: 0 } },
+    }) as { props: { parameters: unknown } }[];
+
+    expect(fill.props.parameters).toEqual(additiveBlendParameters);
+  });
+
+  test("point sublayers receive the additive blend parameters", () => {
+    const layer = buildLayer();
+
+    // Point mode (z < -2): single AdditiveScatterplotLayer.
+    const result = layer.props.renderSubLayers({
+      id: "MarkersLayer-res-1-tile",
+      data: emptyArrowTable,
+      tile: { index: { z: -3, x: 0, y: 0 } },
+    }) as { props: { parameters: unknown } };
+    const points = Array.isArray(result) ? result[0] : result;
+
+    expect(points.props.parameters).toEqual(additiveBlendParameters);
+  });
+
+  test("the additive parameters accumulate source color (src-alpha onto one)", () => {
+    expect(additiveBlendParameters).toMatchObject({
+      blend: true,
+      blendColorOperation: "add",
+      blendColorSrcFactor: "src-alpha",
+      blendColorDstFactor: "one",
+      blendAlphaOperation: "add",
+      blendAlphaSrcFactor: "one",
+      blendAlphaDstFactor: "one",
+    });
+  });
+});
diff --git a/app/components/.client/ImageViewer/components/canvas/Overlays/additiveBlending.glsl.ts b/app/components/.client/ImageViewer/components/canvas/Overlays/additiveBlending.glsl.ts
index 4fe87f14..0fc358c8 100644
--- a/app/components/.client/ImageViewer/components/canvas/Overlays/additiveBlending.glsl.ts
+++ b/app/components/.client/ImageViewer/components/canvas/Overlays/additiveBlending.glsl.ts
@@ -1,3 +1,15 @@
+/** Fragments accumulate onto the framebuffer instead of alpha-blending over it,
+ *  so fills from separate overlay files add up (yellow + blue → white). */
+export const additiveBlendParameters = {
+  blend: true,
+  blendColorOperation: "add",
+  blendColorSrcFactor: "src-alpha",
+  blendColorDstFactor: "one",
+  blendAlphaOperation: "add",
+  blendAlphaSrcFactor: "one",
+  blendAlphaDstFactor: "one",
+} as const;
+
 export const shadersInject = {
   // Vertex shader: declare and pass the marker bitmask
   "vs:#decl": /* glsl */ `
diff --git a/app/components/.client/ImageViewer/components/canvas/Overlays/useOverlaysLayer.tsx b/app/components/.client/ImageViewer/components/canvas/Overlays/useOverlaysLayer.tsx
index e16208b6..61b51bb5 100644
--- a/app/components/.client/ImageViewer/components/canvas/Overlays/useOverlaysLayer.tsx
+++ b/app/components/.client/ImageViewer/components/canvas/Overlays/useOverlaysLayer.tsx
@@ -15,6 +15,7 @@ import {
   type RGBA,
 } from "../../../state/store/types";
 import { useTilesLoading } from "../../../utils/useTilesLoading";
+import { DEFAULT_OVERLAYS_FILL_OPACITY } from "~/utils/overlayDefaults";
 
 /** Creates the overlay marker layers for one image panel. */
 export const useOverlaysLayers = (imagePanelId: number): CytarioLayerResult => {
@@ -29,7 +30,8 @@ export const useOverlaysLayers = (imagePanelId: number): CytarioLayerResult => {
   const imageWidth = metadata?.Pixels?.SizeX ?? 0;
   const imageHeight = metadata?.Pixels?.SizeY ?? 0;
 
-  const fillOpacity = layersStates[panelLayersStateIndex]?.overlaysFillOpacity ?? 0.8;
+  const fillOpacity =
+    layersStates[panelLayersStateIndex]?.overlaysFillOpacity ?? DEFAULT_OVERLAYS_FILL_OPACITY;
   const showCellOutline = layersStates[panelLayersStateIndex]?.showCellOutline ?? true;
 
   const overlayState = layersStates[panelLayersStateIndex]?.overlays ?? null;
diff --git a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/OverlayConfig.modal.tsx b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/OverlayConfig.modal.tsx
index f76ccfc4..a98e617c 100644
--- a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/OverlayConfig.modal.tsx
+++ b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/OverlayConfig.modal.tsx
@@ -1,8 +1,8 @@
 import { Banner, Button, Input, Select, type SelectItem } from "@cytario/design";
-import { useEffect, useMemo, useState } from "react";
+import { useContext, useEffect, useMemo, useState } from "react";
 
-import { getOverlayState, markerDisplayLabel } from "./getOverlayState";
-import { useViewerStore } from "../../../state/store/core/ViewerStoreContext";
+import { getOverlayState, markerDisplayLabel, paletteOffsetFor } from "./getOverlayState";
+import { ViewerStoreContext, useViewerStore } from "../../../state/store/core/ViewerStoreContext";
 import { applyOverlayReconfiguration } from "../../../state/store/overlays/overlays.store";
 import { select } from "../../../state/store/selectors";
 import { type OverlayConfig, type OverlayEntry } from "../../../state/store/types";
@@ -85,6 +85,7 @@ export function OverlayConfigModal({
   onClose,
   onApplyError,
 }: OverlayConfigModalProps) {
+  const viewerStore = useContext(ViewerStoreContext);
   const updateOverlayConfig = useViewerStore(select.updateOverlayConfig);
 
   const [schema, setSchema] = useState<ParquetColumn[] | null>(null);
@@ -177,7 +178,12 @@ export function OverlayConfigModal({
     }
     try {
       const markerInfo = await getMarkerInfoWasm(resourceId, nextConfig);
-      const markers = getOverlayState(markerInfo, nextConfig);
+      const overlaysStates = viewerStore ? select.overlaysStates(viewerStore.getState()) : {};
+      const markers = getOverlayState(
+        markerInfo,
+        nextConfig,
+        paletteOffsetFor(overlaysStates, resourceId),
+      );
       updateOverlayConfig(resourceId, nextConfig, markers);
       applyOverlayReconfiguration(resourceId);
       onClose();
diff --git a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/OverlayItem.tsx b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/OverlayItem.tsx
index eb93e6d7..7beadbca 100644
--- a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/OverlayItem.tsx
+++ b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/OverlayItem.tsx
@@ -1,9 +1,9 @@
 import { Badge, Banner, Link, MenuItem, Switch, Tooltip, useToast } from "@cytario/design";
-import { useEffect, useMemo, useState } from "react";
+import { useContext, useEffect, useMemo, useState } from "react";
 
-import { getOverlayState } from "./getOverlayState";
+import { getOverlayState, paletteOffsetFor } from "./getOverlayState";
 import { OverlayConfigModal } from "./OverlayConfig.modal";
-import { useViewerStore } from "../../../state/store/core/ViewerStoreContext";
+import { useViewerStore, ViewerStoreContext } from "../../../state/store/core/ViewerStoreContext";
 import { select } from "../../../state/store/selectors";
 import { type OverlayEntry, RGBA } from "../../../state/store/types";
 import { AccordionToggle } from "../AccordionToggle";
@@ -26,6 +26,7 @@ interface OverlayItemProps {
 }
 
 export const OverlayItem = ({ resourceId, overlay }: OverlayItemProps) => {
+  const viewerStore = useContext(ViewerStoreContext);
   const setMarkerVisibility = useViewerStore(select.setMarkerVisibility);
   const setMarkerColor = useViewerStore(select.setMarkerColor);
   const removeOverlaysState = useViewerStore(select.removeOverlaysState);
@@ -83,7 +84,12 @@ export const OverlayItem = ({ resourceId, overlay }: OverlayItemProps) => {
       try {
         const markerInfo = await getMarkerInfoWasm(resourceId, overlay.config);
         if (markerInfo && Object.keys(markerInfo).length > 0) {
-          const newOverlayState = getOverlayState(markerInfo, overlay.config);
+          const overlaysStates = viewerStore ? select.overlaysStates(viewerStore.getState()) : {};
+          const newOverlayState = getOverlayState(
+            markerInfo,
+            overlay.config,
+            paletteOffsetFor(overlaysStates, resourceId),
+          );
           updateOverlaysState(resourceId, newOverlayState);
         } else {
           toast({
@@ -111,6 +117,7 @@ export const OverlayItem = ({ resourceId, overlay }: OverlayItemProps) => {
     toast,
     fileName,
     overlay.config,
+    viewerStore,
   ]);
 
   // Total cell/object count for the file-level badge (rows in the parquet).
diff --git a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/__tests__/OverlayConfig.modal.test.tsx b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/__tests__/OverlayConfig.modal.test.tsx
index a82d6e82..367f7406 100644
--- a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/__tests__/OverlayConfig.modal.test.tsx
+++ b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/__tests__/OverlayConfig.modal.test.tsx
@@ -8,9 +8,11 @@ import { OverlayConfigModal } from "../OverlayConfig.modal";
 import { getParquetTopLevelSchema } from "~/components/DataGrid/getParquetSchema";
 import { getMarkerInfoWasm } from "~/utils/db/getMarkerInfoWasm";
 
-vi.mock("../../../../state/store/core/ViewerStoreContext", () => ({
-  useViewerStore: vi.fn(),
-}));
+vi.mock("../../../../state/store/core/ViewerStoreContext", async (importOriginal) => {
+  const actual =
+    await importOriginal<typeof import("../../../../state/store/core/ViewerStoreContext")>();
+  return { ...actual, useViewerStore: vi.fn() };
+});
 
 vi.mock("~/components/DataGrid/getParquetSchema", () => ({
   getParquetSchema: vi.fn(),
diff --git a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/__tests__/getOverlayState.test.ts b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/__tests__/getOverlayState.test.ts
index ef491017..4e49340d 100644
--- a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/__tests__/getOverlayState.test.ts
+++ b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/__tests__/getOverlayState.test.ts
@@ -1,4 +1,6 @@
-import { getOverlayState, markerDisplayLabel } from "../getOverlayState";
+import type { OverlaysState } from "../../../../state/store/types";
+import { CATEGORICAL_COLORS } from "../../SectionRow/ColorPicker/utils";
+import { getOverlayState, markerDisplayLabel, paletteOffsetFor } from "../getOverlayState";
 import type { OverlayConfig } from "~/utils/db/overlayConfig";
 
 const config: OverlayConfig = {
@@ -49,3 +51,47 @@ describe("getOverlayState with config", () => {
     expect(state["marker_positive_cd4"].label).toBe("cd4");
   });
 });
+
+describe("palette offset across overlays", () => {
+  const makeMarker = () => ({
+    color: [0, 0, 0, 1] as [number, number, number, number],
+    count: 0,
+    isVisible: false,
+    label: "",
+  });
+
+  const makeOverlays = (markerCounts: Record<string, number>) =>
+    Object.fromEntries(
+      Object.entries(markerCounts).map(([id, markerCount]) => [
+        id,
+        {
+          markers: Object.fromEntries(
+            Array.from({ length: markerCount }, (_, i) => [`marker_${i}`, makeMarker()]),
+          ),
+          config: null,
+        },
+      ]),
+    ) as unknown as OverlaysState;
+
+  test("counts only markers from other overlays", () => {
+    const overlays = makeOverlays({ "res-a": 3, "res-b": 2, "res-c": 4 });
+
+    expect(paletteOffsetFor(overlays, "res-b")).toBe(7);
+    expect(paletteOffsetFor(overlays, "res-a")).toBe(6);
+    expect(paletteOffsetFor({}, "res-a")).toBe(0);
+  });
+
+  test("offset shifts the palette so same-named markers land on distinct colors", () => {
+    const first = getOverlayState({ marker_positive_ck: { count: 1 } });
+    const second = getOverlayState({ marker_positive_ck: { count: 1 } }, null, 1);
+
+    expect(first["marker_positive_ck"].color).toEqual(CATEGORICAL_COLORS[0]);
+    expect(second["marker_positive_ck"].color).toEqual(CATEGORICAL_COLORS[1]);
+  });
+
+  test("offset wraps around the palette", () => {
+    const state = getOverlayState({ a: { count: 1 } }, null, CATEGORICAL_COLORS.length + 1);
+
+    expect(state["a"].color).toEqual(CATEGORICAL_COLORS[1]);
+  });
+});
diff --git a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/getOverlayState.ts b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/getOverlayState.ts
index f2f7fa65..5d327b20 100644
--- a/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/getOverlayState.ts
+++ b/app/components/.client/ImageViewer/components/sidebar/OverlaysSection/getOverlayState.ts
@@ -1,4 +1,4 @@
-import { OverlayState, RGBA } from "../../../state/store/types";
+import { OverlaysState, OverlayState, RGBA } from "../../../state/store/types";
 import { CATEGORICAL_COLORS } from "../SectionRow/ColorPicker/utils";
 import { type OverlayConfig, MARKER_POSITIVE_PREFIX } from "~/utils/db/overlayConfig";
 
@@ -8,6 +8,14 @@ export type MarkerInfo = Record<string, { count: number }>;
 const categoricalColorAt = (index: number): RGBA =>
   CATEGORICAL_COLORS[index % CATEGORICAL_COLORS.length];
 
+/** Markers held by overlays other than {@link resourceId}, shifted past so the
+ *  same marker loaded from two files starts life on distinct palette colors. */
+export function paletteOffsetFor(overlays: OverlaysState, resourceId: string): number {
+  return Object.entries(overlays)
+    .filter(([id]) => id !== resourceId)
+    .reduce((offset, [, entry]) => offset + Object.keys(entry.markers).length, 0);
+}
+
 /** Display label for a marker: config label wins, else the prefix-stripped key. */
 export function markerDisplayLabel(markerKey: string, config?: OverlayConfig | null): string {
   const cls = config?.classes.find((c) => c.sourceColumn === markerKey);
@@ -20,11 +28,12 @@ export function markerDisplayLabel(markerKey: string, config?: OverlayConfig | n
 export function getOverlayState(
   markerInfo: MarkerInfo,
   config?: OverlayConfig | null,
+  paletteOffset = 0,
 ): OverlayState {
   const columnNames = Object.keys(markerInfo);
   const overlayState = columnNames.reduce((acc, name, index) => {
     acc[name] = {
-      color: categoricalColorAt(index),
+      color: categoricalColorAt(index + paletteOffset),
       count: markerInfo[name].count ?? 0,
       isVisible: false,
       label: markerDisplayLabel(name, config),
diff --git a/app/components/.client/ImageViewer/state/store/selectors.ts b/app/components/.client/ImageViewer/state/store/selectors.ts
index 6d95a136..420936a3 100644
--- a/app/components/.client/ImageViewer/state/store/selectors.ts
+++ b/app/components/.client/ImageViewer/state/store/selectors.ts
@@ -9,6 +9,7 @@ import {
   LayerChannelsState,
   ViewerStore,
 } from "./types";
+import { DEFAULT_OVERLAYS_FILL_OPACITY } from "~/utils/overlayDefaults";
 
 const EMPTY_OBJECT = Object.freeze({});
 
@@ -244,7 +245,7 @@ export const select = {
 
   overlaysFillOpacity: (state: ViewerStore) => {
     const layerState = getLayersState(state);
-    return layerState?.overlaysFillOpacity ?? 0.8;
+    return layerState?.overlaysFillOpacity ?? DEFAULT_OVERLAYS_FILL_OPACITY;
   },
   setOverlaysFillOpacity: (state: ViewerStore) => state.setOverlaysFillOpacity,
 
diff --git a/app/components/.client/ImageViewer/state/store/types.ts b/app/components/.client/ImageViewer/state/store/types.ts
index 63309f50..67255c1c 100644
--- a/app/components/.client/ImageViewer/state/store/types.ts
+++ b/app/components/.client/ImageViewer/state/store/types.ts
@@ -9,6 +9,7 @@ import type { ViewSlice } from "./core/viewport.store";
 import type { OverlaysSlice } from "./overlays/overlays.store";
 import type { ViewsSlice } from "./views/views.store";
 import type { OverlayConfig } from "~/utils/db/overlayConfig";
+import { DEFAULT_OVERLAYS_FILL_OPACITY } from "~/utils/overlayDefaults";
 
 export type { OverlayConfig };
 
@@ -84,7 +85,7 @@ export const createDefaultLayersStateEntry = (author = ""): LayersStateEntry =>
   channels: {},
   overlays: {},
   channelsOpacity: 1,
-  overlaysFillOpacity: 0.5,
+  overlaysFillOpacity: DEFAULT_OVERLAYS_FILL_OPACITY,
   showCellOutline: true,
   annotationsOpacity: 0.5,
   showAnnotationOutline: true,
diff --git a/app/utils/__tests__/overlayDefaults.test.ts b/app/utils/__tests__/overlayDefaults.test.ts
new file mode 100644
index 00000000..01f8e9c4
--- /dev/null
+++ b/app/utils/__tests__/overlayDefaults.test.ts
@@ -0,0 +1,31 @@
+import { viewSettingsDocumentSchema } from "../db/viewSettingsSchema";
+import { DEFAULT_OVERLAYS_FILL_OPACITY } from "../overlayDefaults";
+import { select } from "~/components/.client/ImageViewer/state/store/selectors";
+import { createDefaultLayersStateEntry } from "~/components/.client/ImageViewer/state/store/types";
+import type { ViewerStore } from "~/components/.client/ImageViewer/state/store/types";
+
+describe("DEFAULT_OVERLAYS_FILL_OPACITY", () => {
+  test("is semi-transparent", () => {
+    expect(DEFAULT_OVERLAYS_FILL_OPACITY).toBeGreaterThan(0);
+    expect(DEFAULT_OVERLAYS_FILL_OPACITY).toBeLessThan(1);
+  });
+
+  test("seeds a new layers state entry", () => {
+    expect(createDefaultLayersStateEntry().overlaysFillOpacity).toBe(DEFAULT_OVERLAYS_FILL_OPACITY);
+  });
+
+  test("applies to sidecar view entries that omit the field", () => {
+    const document = viewSettingsDocumentSchema.parse({
+      cytario: { schemaVersion: "1.1", kind: "settings", image: "img.ome.tiff", author: "a" },
+      views: [{ id: "view-1", author: "a", channels: {} }],
+    });
+    expect(document.views[0].overlaysFillOpacity).toBe(DEFAULT_OVERLAYS_FILL_OPACITY);
+  });
+
+  test("is the selector fallback when no layers state exists", () => {
+    const emptyStore = { imagePanels: [0], imagePanelIndex: 0, layersStates: [] };
+    expect(select.overlaysFillOpacity(emptyStore as unknown as ViewerStore)).toBe(
+      DEFAULT_OVERLAYS_FILL_OPACITY,
+    );
+  });
+});
diff --git a/app/utils/db/viewSettingsSchema.ts b/app/utils/db/viewSettingsSchema.ts
index 5f0fd2e0..51f8d138 100644
--- a/app/utils/db/viewSettingsSchema.ts
+++ b/app/utils/db/viewSettingsSchema.ts
@@ -5,6 +5,7 @@ import type {
   LayersStateEntry,
   RGB,
 } from "~/components/.client/ImageViewer/state/store/types";
+import { DEFAULT_OVERLAYS_FILL_OPACITY } from "~/utils/overlayDefaults";
 
 const SCHEMA_VERSION = "1.1";
 
@@ -70,7 +71,7 @@ export const viewSettingsChannelSchema = z.object({
   ),
   channelsOpacity: z.number().default(1),
   overlays: sidecarOverlaysSchema.default({}),
-  overlaysFillOpacity: z.number().default(0.8),
+  overlaysFillOpacity: z.number().default(DEFAULT_OVERLAYS_FILL_OPACITY),
   showCellOutline: z.boolean().default(true),
   annotationsOpacity: z.number().default(1),
   showAnnotationOutline: z.boolean().default(true),
diff --git a/app/utils/overlayDefaults.ts b/app/utils/overlayDefaults.ts
new file mode 100644
index 00000000..a7ff34fe
--- /dev/null
+++ b/app/utils/overlayDefaults.ts
@@ -0,0 +1,2 @@
+/** Semi-transparent so fills from separate overlay sets stay distinguishable where they overlap. */
+export const DEFAULT_OVERLAYS_FILL_OPACITY = 0.5;