what to use to write on photos?

I recently had prints done from mpix and they are on a glossy type of paper. The customers i am selling them to would like them signed. What type of pen should i use to sign these? thanks

silver or black sharpie, find point prefered

Thank you, that was my guess but i just wanted to be sure as to not ruin a print.

theres also some silver gel pens that work well, as they have less chance of spreading, but they're hard to see on a color print

Thanks, im going to go with a fine point black sharpie as they are color prints.