There's three controls on the Blender node: two colour boxes and a numeric box. You would have one colour box set to white, and connect the normal texture. The other would be set to the tattoo colour. The numeric box defaults to 0.5. It should be set to 1, and the tattoo "control" texture connected. That numeric box default still catches me sometimes. You might get slightly better results with a value slightly less than 1, expecially if you have some variations of skin-tone in the tattooed area, and the tattoo colour is a bit lighter.