Fix: GPv3: Flip primary and secondary toggle and shortcut missing

The `paint.brush_colors_flip` option was not exposed in the vertex color
panel.

This adds the toggle in the panel and also adds the keymap `X` to flip
the colors.

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -3745,7 +3745,8 @@ def km_grease_pencil_paint_mode(params):
 
         # Show/hide
         *_template_items_hide_reveal_actions("grease_pencil.layer_hide", "grease_pencil.layer_reveal"),
-
+        # Flip primary and secondary color
+        ("paint.brush_colors_flip", {"type": 'X', "value": 'PRESS'}, None),
         ("paint.sample_color", {"type": 'X', "value": 'PRESS', "shift": True}, None),
 
         # Isolate Layer
@@ -4021,6 +4022,9 @@ def km_grease_pencil_vertex_paint(params):
          {"properties": [("data_path", "scene.tool_settings.use_gpencil_vertex_select_mask_stroke")]}),
         ("wm.context_toggle", {"type": 'THREE', "value": 'PRESS'},
          {"properties": [("data_path", "scene.tool_settings.use_gpencil_vertex_select_mask_segment")]}),
+        # Flip primary and secondary color
+        ("paint.brush_colors_flip", {"type": 'X', "value": 'PRESS'}, None),
+
         # Radial controls
         *_template_paint_radial_control("gpencil_vertex_paint"),
         # Context menu

scripts/startup/bl_ui/space_view3d.py

@@ -8586,8 +8586,7 @@ class TOPBAR_PT_grease_pencil_vertex_color(Panel):
         UnifiedPaintPanel.prop_unified_color(sub_row, context, brush, "color", text="")
         UnifiedPaintPanel.prop_unified_color(sub_row, context, brush, "secondary_color", text="")
 
-        # TODO
-        # sub_row.operator("gpencil.tint_flip", icon='FILE_REFRESH', text="")
+        sub_row.operator("paint.brush_colors_flip", icon='FILE_REFRESH', text="")
 
         row = layout.row(align=True)
         row.template_ID(paint, "palette", new="palette.new")

source/blender/editors/sculpt_paint/paint_image.cc

@@ -1062,6 +1062,11 @@ static bool brush_colors_flip_poll(bContext *C)
       if (ob->mode & (OB_MODE_VERTEX_PAINT | OB_MODE_TEXTURE_PAINT | OB_MODE_SCULPT)) {
         return true;
       }
+      if (blender::ed::greasepencil::grease_pencil_painting_poll(C) ||
+          blender::ed::greasepencil::grease_pencil_vertex_painting_poll(C))
+      {
+        return true;
+      }
     }
   }
   return false;