going over black ink?

can you go over black ink with another color, even white?

Most "pros" will tell you no, but I have done some weird work on a few people using white and colors over black for a murky design. What is the design in black, and what do you want over it?

the design is of the full moon, and it's mostly gray with some black and very little white, and it's about an inch in diameter. if it's not worth it to go over it, i'm going to just leave it, but if possible, then i'd like to go over with a celtic design of the tree of life (just the middle part of this picture: http://radio.weblogs.com/0126951/images/myPictures/2004/05/25/Celtic%20Tree%20of%20Life.GIF).

also, the moon i have is sort of new-agey but i think i would rather have something more elegant, like a celtic design.