If the weight map is over 0, the skin turns lighter and lighter. At 1, the skin is white.

I have a skin there with the tattoo on the skin- the tattoo is black. It cuts out a section of the white - the shape of the black tattoo.

Looks like another thing I will have to give up trying to do. Something as simple as putting a certain color tattoo on a skin shouldn't be this hard. I don't understand why a black tattoo on skin shows up red in the render, and why a black lie even renders red.