Add color blending modes like color erase, multiply, screen, soft light, etc. for brushes to PMCSkin3d, like what you could find in Krita or Photoshop. A few open source implementations of this already exist. The options for it could probably be another tab under the brush settings.
I would like this feature because it would be extremely helpful for color correction and shading, and would also significantly reduce the amount of times I have to export my skin to external programs in order to make adjustments.