Typically tooling gelcoats are done in a colour that is likely to contrast well, so bright greens/oranges/reds etc. Makes it easier to spray an opaque gelcoat into and identify poor coverage.

Really only a consideration when you are fibreglassing with a gelcoat. If you're using carbon, with either no gelcoat or a clear gelcoat, then it isn't very important. Personally, I don't like black gelcoats as much because it makes it slightly harder to see if you have gaps between plies of carbon, but you dont often miss these anyway so even that isn't a huge concern.