If you are going to use it I would question if a gold or silver would hold up well. I suspect black would be more durable but that is only a guess. I would have it signed in a location that gets as little wear as possible.
I haven't had much experience in getting guitars signed, but Blue Sharpie pens work well on baseball bats, which seem they'd have similar finishes. If you really want to get it done in silver or gold, I would look into a Sakura paint pen instead of Sharpie. The pigments seem denser on their permanent pens, and their paint pens seem to hold up well for art.
Well, thanks for feedback. Black or blue Sharpies are out of question as the guitar itself is black so only silver/gold would look good on it... I think I might look into Sakura paint pens... Would you recommend 1mm or 2mm for guitar?
The 1mm writes thin, however, I would probably still lean toward that, as the 2mm marker might be more likely to smear. Remember, paint pens take longer to dry, so account for the extra drying time. The offset is that they present a lot better and last longer than other types of pigments.