Special colorants are designed to tint gelcoat to a desired color tone. Never use colorants that do not specifically state that they are designed for use in gelcoat. If unsure, ask the gelcoat manufacturer which colorants are acceptable. Generally, you would start with either a white or neutral gelcoat base, then add the tints until you achieve the desired color. Always tint the gelcoat prior to adding MEKP, or there may not be enough working time to apply the gelcoat before it begins to cure. Gelcoats also have a maximum concentration of colorant that can be added to retain proper cured properties—this is usually 5-20%, and it varies by manufacturer.
When color matching, the color of uncured gelcoat shifts when it cures, so you must always cure, wet-sand, then buff a color sample to see if it matches properly.
IMPORTANT: Again, never use colorants that do not specifically state that they are designed for use in gelcoat. It's a good idea to ask the gelcoat manufacturer which colorants are acceptable if you are not sure. If colorants are added in too high of a concentration, they can have a negative effect on the properties of the cured gelcoat.