I understood that last sentence the best, Bill, because it was pretty much what I had suspected. The edges of the lips were defined by the borders of the mesh and material zone, so yeah, hard to get blending happening there.

The second-last sentence also seems fairly logical and is a major source of puzzlement to me: how a mask could influence shaders. I suppose the bottom of the tree would involve a transparent zone (kinda like doing transmapped hair) and then you build your shader tree on that?

I puzzled on that second sentence a lot longer. The underlying texture - the wrinkles and stuff in lip tissue - would be defined by the underlying skin: that's assuming that you've got a decent texture to start with. Then, what, the colour - carefully doing maths because of energy conservation - and then the gloss - more careful maths because of diffuse and gloss needing to play together, all over the basic skin and the lip texture? Does my sandwich have enough layers yet? Oh, and I'm not even considering SSS and AO or any of that stuff. 20 nodes? Wouldn't have a clue where to start.

That first sentence summed it all up perfectly, and brought it all into synch with what you and the other bright kids on this thread were discussing.

Upshot is: I'll leave off make-up. This character is going to be a natural girl. There is no mixing and matching VSS and traditional make-up kits that you paint on to your nude skin texture. It's one or the other.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks