My 22yr old daughter has colored tattoos. What kind of ink was used to make tattoos years ago?

My 84yr old father got tattoos when he signed up to join the army, for WWII. His tattoos now look like it is greenish in appearance, not black ink. So, I'd like to know if someone can give me an answer!
Even when I was a child, his tattoos always looked green! Is it the ink? Or is it because where it was done?