scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -4586,6 +4586,8 @@ def km_grease_pencil_paint(_params):
 
         # Show/hide
         *_template_items_hide_reveal_actions("grease_pencil.layer_hide", "grease_pencil.layer_reveal"),
+
+        ("paint.sample_color", {"type": 'X', "value": 'PRESS', "shift": True}, None),
     ])
 
     return keymap

scripts/startup/bl_ui/space_view3d.py

@@ -2179,6 +2179,10 @@ class VIEW3D_MT_paint_grease_pencil(Menu):
 
         layout.menu("VIEW3D_MT_edit_greasepencil_showhide")
 
+        layout.separator()
+
+        layout.operator("paint.sample_color")
+
 
 class VIEW3D_MT_paint_gpencil(Menu):
     bl_label = "Paint"

source/blender/editors/sculpt_paint/paint_image.cc

@@ -52,6 +52,7 @@
 #include "UI_interface.hh"
 #include "UI_view2d.hh"
 
+#include "ED_grease_pencil.hh"
 #include "ED_image.hh"
 #include "ED_object.hh"
 #include "ED_paint.hh"
@@ -760,7 +761,8 @@ static int sample_color_modal(bContext *C, wmOperator *op, const wmEvent *event)
 
 static bool sample_color_poll(bContext *C)
 {
-  return (image_paint_poll_ignore_tool(C) || vertex_paint_poll_ignore_tool(C));
+  return (image_paint_poll_ignore_tool(C) || vertex_paint_poll_ignore_tool(C) ||
+          blender::ed::greasepencil::grease_pencil_painting_poll(C));
 }
 
 void PAINT_OT_sample_color(wmOperatorType *ot)