OK here's the tut. I can think of other ways to do this, possibly keeping even more skin color variation. Follow along and see what you get. If you want a more advanced way of keeping more skin texture, come back and ask. It's just a couple more nodes.

First, open the skin shader for where you want the tattoo. I'm using the hip.

I'm using V4 because it is pretty complicated, so you'll see how to handle this case.

Track down where the color map is going. In the V4 shader, it goes to two places. I opened the node so you can see where it goes.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)