How do I paint the vertex color of a source mesh (not an instance)

I have some bushes and I want to vertex paint them to adjust them to the wind. I can paint the instances, but there are thousands in my level and they are instantiated as grass or foliage.
I can see the vertex in the mesh viewer, but I cannot paint them.
Is there a tool or a way to paint these vertex for the source mesh, not the instances, so that when the source is spawned, all the instances inherit of the vertex colors?

The easiest way I find is to pain the vertices in your 3D modelling software and import them into unreal with the vertex colour option set to “replace”. You can paint them in editor though, it’s just not quite as clean - paint an instance of the mesh in-level, the hit this button

That will propagate the vertex colour to the source mesh