diff --git a/x-pack/platform/plugins/shared/lens/public/visualizations/xy/color_assignment.test.ts b/x-pack/platform/plugins/shared/lens/public/visualizations/xy/color_assignment.test.ts
index d7252f2b9977c..a368857613fec 100644
--- a/x-pack/platform/plugins/shared/lens/public/visualizations/xy/color_assignment.test.ts
+++ b/x-pack/platform/plugins/shared/lens/public/visualizations/xy/color_assignment.test.ts
@@ -5,7 +5,11 @@
  * 2.0.
  */
 
-import { getAssignedColorConfig, getColorAssignments } from './color_assignment';
+import {
+  getAssignedColorConfig,
+  getColorAssignments,
+  getLayerPaletteName,
+} from './color_assignment';
 import type { FormatFactory } from '../../../common/types';
 import { LayerTypes } from '@kbn/expression-xy-plugin/public';
 import type { XYDataLayerConfig } from './types';
@@ -230,6 +234,28 @@ describe('color_assignment', () => {
     });
   });
 
+  describe('getLayerPaletteName', () => {
+    it('should fall back to line-optimized palette for line series with no explicit palette', () => {
+      const layer: XYDataLayerConfig = {
+        ...layers[0],
+        seriesType: 'line',
+        palette: undefined,
+        colorMapping: undefined,
+      };
+      expect(getLayerPaletteName(layer)).toEqual(KbnPalette.ElasticLineOptimized);
+    });
+
+    it('should fall back to default palette for bar series with no explicit palette', () => {
+      const layer: XYDataLayerConfig = {
+        ...layers[0],
+        seriesType: 'bar',
+        palette: undefined,
+        colorMapping: undefined,
+      };
+      expect(getLayerPaletteName(layer)).toEqual(KbnPalette.Default);
+    });
+  });
+
   describe('colorMapping palette support', () => {
     it('should group layers by colorMapping.paletteId when present', () => {
       const lineLayer: XYDataLayerConfig = {
@@ -318,5 +344,45 @@ describe('color_assignment', () => {
       expect(paletteService.get).toHaveBeenCalledWith('default');
       expect(defaultGetCategoricalColor).toHaveBeenCalled();
     });
+
+    it('should group bar layers under the default palette', () => {
+      const barLayer1: XYDataLayerConfig = {
+        ...layers[0],
+        layerId: '1',
+        seriesType: 'bar',
+        palette: undefined,
+        colorMapping: undefined,
+      };
+      const barLayer2: XYDataLayerConfig = {
+        ...layers[1],
+        layerId: '2',
+        seriesType: 'bar',
+        palette: undefined,
+        colorMapping: undefined,
+      };
+      const assignments = getColorAssignments([barLayer1, barLayer2], data, formatFactory);
+      expect(assignments[KbnPalette.ElasticLineOptimized]).toBeUndefined();
+      expect(assignments[KbnPalette.Default]).toBeDefined();
+    });
+
+    it('should group multiple line layers under the line-optimized palette', () => {
+      const lineLayer1: XYDataLayerConfig = {
+        ...layers[0],
+        layerId: '1',
+        seriesType: 'line',
+        palette: undefined,
+        colorMapping: undefined,
+      };
+      const lineLayer2: XYDataLayerConfig = {
+        ...layers[1],
+        layerId: '2',
+        seriesType: 'line',
+        palette: undefined,
+        colorMapping: undefined,
+      };
+      const assignments = getColorAssignments([lineLayer1, lineLayer2], data, formatFactory);
+      expect(assignments[KbnPalette.ElasticLineOptimized]).toBeDefined();
+      expect(assignments[KbnPalette.Default]).toBeUndefined();
+    });
   });
 });
diff --git a/x-pack/platform/plugins/shared/lens/public/visualizations/xy/color_assignment.ts b/x-pack/platform/plugins/shared/lens/public/visualizations/xy/color_assignment.ts
index baed001b72966..e2bbe024b3d8f 100644
--- a/x-pack/platform/plugins/shared/lens/public/visualizations/xy/color_assignment.ts
+++ b/x-pack/platform/plugins/shared/lens/public/visualizations/xy/color_assignment.ts
@@ -25,13 +25,14 @@ import {
   getSingleColorConfig,
 } from './reference_line_helpers';
 import type { XYDataLayerConfig, XYLayerConfig } from './types';
+import { getDefaultPalette } from './default_palette';
 
 const isPrimitive = (value: unknown): boolean => value != null && typeof value !== 'object';
 
 export const defaultReferenceLineColor = euiLightVars.euiColorDarkShade;
 
 export const getLayerPaletteName = (layer: XYDataLayerConfig): string =>
-  layer.colorMapping?.paletteId ?? layer.palette?.name ?? 'default';
+  layer.colorMapping?.paletteId ?? layer.palette?.name ?? getDefaultPalette(layer.seriesType);
 
 const getPaletteDefinition = (paletteService: PaletteRegistry, paletteName: string) =>
   paletteService.get(paletteName) ?? paletteService.get('default');
diff --git a/x-pack/platform/plugins/shared/lens/public/visualizations/xy/visualization.tsx b/x-pack/platform/plugins/shared/lens/public/visualizations/xy/visualization.tsx
index ad76b6d17af03..4f5e6fe7077c7 100644
--- a/x-pack/platform/plugins/shared/lens/public/visualizations/xy/visualization.tsx
+++ b/x-pack/platform/plugins/shared/lens/public/visualizations/xy/visualization.tsx
@@ -78,6 +78,7 @@ import {
   getColorAssignments,
   getLayerPaletteName,
 } from './color_assignment';
+import { getDefaultPalette } from './default_palette';
 import {
   getAnnotationLayerErrors,
   isHorizontalChart,
@@ -494,7 +495,9 @@ export const getXyVisualization = ({
         })
         .unsubscribe();
     } else {
-      const palette = paletteService.get(dataLayer.palette?.name || 'default');
+      const palette = paletteService.get(
+        dataLayer.palette?.name || getDefaultPalette(dataLayer.seriesType)
+      );
       colors = palette.getCategoricalColors(10, dataLayer.palette?.params);
     }
 
diff --git a/x-pack/platform/plugins/shared/lens/public/visualizations/xy/xy_config_panel/dimension_editor.tsx b/x-pack/platform/plugins/shared/lens/public/visualizations/xy/xy_config_panel/dimension_editor.tsx
index 242ee128536bf..024fdf0d53624 100644
--- a/x-pack/platform/plugins/shared/lens/public/visualizations/xy/xy_config_panel/dimension_editor.tsx
+++ b/x-pack/platform/plugins/shared/lens/public/visualizations/xy/xy_config_panel/dimension_editor.tsx
@@ -12,7 +12,7 @@ import { ColorPicker } from '@kbn/visualization-ui-components';
 
 import { EuiButtonGroup, EuiFormRow, htmlIdGenerator } from '@elastic/eui';
 import type { PaletteRegistry, ColorMapping, PaletteOutput } from '@kbn/coloring';
-import { canCreateCustomMatch } from '@kbn/coloring';
+import { DEFAULT_COLOR_MAPPING_CONFIG, canCreateCustomMatch } from '@kbn/coloring';
 import { getColorCategories } from '@kbn/chart-expressions-common';
 import type { ValuesType } from 'utility-types';
 import type { KbnPalettes } from '@kbn/palettes';
@@ -31,6 +31,7 @@ import {
   getAssignedColorConfig,
   getLayerPaletteName,
 } from '../color_assignment';
+import { getDefaultPalette } from '../default_palette';
 import { ColorMappingByTerms } from '../../../shared_components/coloring/color_mapping_by_terms';
 
 export const idPrefix = htmlIdGenerator()();
@@ -170,11 +171,15 @@ export function DataDimensionEditor(
       formatter = props.formatFactory(columnMeta?.params);
     }
 
+    const defaultColorMapping = !layer.palette
+      ? { ...DEFAULT_COLOR_MAPPING_CONFIG, paletteId: getDefaultPalette(layer.seriesType) }
+      : undefined;
+
     return !layer.collapseFn ? (
       <div className="lnsIndexPatternDimensionEditor--padded">
         <ColorMappingByTerms
           isDarkMode={isDarkMode}
-          colorMapping={layer.colorMapping}
+          colorMapping={layer.colorMapping ?? defaultColorMapping}
           palette={layer.palette}
           isInlineEditing={isInlineEditing}
           setPalette={setPalette}