Is it possible to add the mask only on the white material of the shoe, but so that the sole is still visible? Look the first picture
I ´ve had a similar problem and couldn´t find a solution. I think i ´ve read somewhere, that currently a mask is always applied to all layers, so a nice feature request would be to apply unique masks to a single layer.
The Solution for my problem was, that i ve have made an inverted PNG, where i punched out the area that should be writeable by the customer and added a fill everywhere else. The PNG was set as an extra layer which always stays on top. Maybe something like this will help you too!?
Anyway: +1 for individual masks. :)