Remember tattoos are an art form and yours belong to you, with that said, if you want to add color to a finished tattoo, by all means, ask your artist if it can be done. Of course, you can‘t put color over solid black but you can add color around it to have it appear as it were part of the original design.
Regarding this, do tattoos have to match?
Yes it’s ok to mix. As long as the work is of good quality you can mix it up. Good tattoos look fine next to other good tattoos regardless of styles. I have some traditional next to some black and grey next to a little Japanese.