update: solved, kind of. not worth investigating because advanced skeleton actually has a blendshape correction wizard, which if you use, makes it so that this problem isn't a problem. I had been adjusting the blendshapes manually.
if using sculpting brushes on a non-skinned model, you can toggle the display so that the textures appear
but if I sculpt on a skinned model (e.g. for blendshapes), then this gray color is forced on.
If I toggle the Show Frozen/masked, that will reenable the texture view, but as soon as I apply the brush it forces the gray color back on.
This is a bug right? Anybody experienced it, found a workaround? It's nice to not have to toggle back and forth out of the brush to see effect on the textured model.
one important note, might be related to advanced skeleton, which is creating some color set nodes. not sure what it means exactly but similar issues online are related to such nodes.