Change Grease Pencil's Vertex Color swatch to a different icon when using Material Color

In Grease Pencil, you can select either Material Color or Vertex Color in the tool settings header. When Material Color is selected, the Vertex Color swatch "grays out" to show it's not in use, and you can't click it to change it. This is visually unclear. Instead of visually expressing "disabled," it changes from a green color to a lighter green color, thereby still looking like a chosen color.

My proposal is that the Vertex Color swatch should have a disabled icon overlay when Material Color is selected, such as a Lock or an X.