It seems mask fail when you look at them laterally. Is this a issue?
I am trying to colour fake stones, which when using the colour options changes the shapes because it uses said colours as a mask it seems when plugged into a child of a surface layer (darker the colour, the more the layer below shows). So I kept them bright white, and then used them as a mask to colour them.
At close range looking down at them it works fine, 100% of the rocks are covered, however at a distance, the mask seems to fail the more you look at it from the side.