Board Thread:Questions and Answers/@comment-202.166.19.138-20151223040540/@comment-25955276-20160124231607

It's hard to tell how Uta's tattoos stay, though it could be that the ghoul anatomy has some similar parts to human anatomy. With the tattoonig process, it could be that Uta used a machine made out of quinque, which would make more sense than him using RC inhibitors seeing as that would be that he would be vulnerable to attack for some hours.

For humans, scientifically, the tattoos stay because ink has been injected into the dermis (the area under skin), and cells that come to eat up the ink because it's a foreign body get dyed by the ink. The cells die, and other cells like it will eat it up, those cells being dyed as well.