A mask is an image where you are defining the strength of an effect. In this case, you're concerned with the fact that you are making a texture set, with colors, and these colors include the color of makeup. It doesn't really matter what color that is, the important thing is that it blocks the light from reaching the skin. It diffusely reflects off the makeup first. Examples would be eyeliner, which completely blocks scatter, lipstick can be total block or partial, eye shadow is usually partially blocking light from reaching the skin.

If any of these are dark, they will block scatter more just by being darker colors.

However, if they are light, but supposed to be opaque, you would want to indicate that on those areas there is less scatter.

A mask should be made where you draw white where the skin scatters most, gray where it scatters less, and black where it does not scatter at all.

Then you'd use this mask to blend two nodes - an ordinary Diffuse node, and a Scatter node. Both would receive the color from the color map, but which one gets to actually emit its output to the render will be controlled by the Blender node. The Blender Blending value would come from the scatter map.

When the mask is 0 (black), the Diffuse node would take the whole job.

When the mask is 1 (white), the Scatter node would take the whole job.

Any values in between (grays) would indicate a mixture or blending of the two nodes.

It is important that the effect of this mask is always the same, so when using it, set its gamma to 1. It is not a picture. It is data.

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