If you have some gelcoat discoloration due to sun degredation, for instance, then you can wet sand the gelcoat to expose gelcoat that has not been exposed to the sun, but you have to be careful that you don't sand through the gelcoat revealing the lamination beneath, which will create another problem. Or, if you are patching an area with gelcoat and are trying to get a good match, this takes patience. You have to add various drops of color to find the color you are trying to match. If you want it lighter, add white. Darker, add black (but not much). Sometimes you have to add a little yellow, (red and yellow make brown), not sure what color you are trying to achieve, but it takes trial and error and patience.