Not all gelcoat is white. You can tint it much like any other coating.
I didn't realize it was common practice to tint the gelcoat of a boat's hull below the waterline. In fact, I'm not sure I've ever heard of that in a sailboat. I'm certainly not an authority on boat building, though, so take my comments with a grain of salt. (or speck of gelcoat dust, as the case may be)
BTW, if you're sanding the boat and wore a proper respirator mask, you wouldn't be sneezing gelcoat.
I wore some safety goggles and a disposable dust thingy over my nose/mouth, same as all the guys in the yard did, but nothing fancy. Don't think I would recommend doing your own bottom work even if I did it in full pressure diving gear, though. It's just not particularly fun.
Like I said, I'm glad I did it, so I know how it's done, and I bet I did a better job than many others would have done, but I'm not doing it again. That was a week I'd rather spend doing something else.