As far as doing it with nodes, I would have just a tattoo image and a mask of it to work with. The idea is to place it to look under the skin so to speak. I would prefer a monochrome to a color if it made it easier to have it look in the skin as opposed to on the skin.

But "on the skin brings up an other issue... I once was able to make grease paint, such as a white clown face, that did look on the skin as opposed to the skin simply changed in color, but now I can't recreate the effect, I can't rember how I got the look of opaque white over my texture but retaing the texture look not just with a bump map but on the texture image as well.