Color matching gelcoat sounds like it would be as easy as matching a paint color at your local hardware store, but it is unfortunately an extremely difficult thing to do.
Gelcoat naturally fades and oxidizes over time, making darker, more bold colors muted, and bright white colors can yellow or become an off white and lose their real finish ‘pop’. Whether the gelcoat is two years old or 30 years old, when you go to repair it, the color is not the same as it was originally, which makes matching that color tone more difficult. Gelcoat colorants are very concentrated, so it may take mere pin-drops of colorant to change a color dramatically.
The best way to try matching a color is start with the right base color of gelcoat, and then a full selection of tints. Gelcoat generally comes in white or neutral bases. White is used for white or off-white tones, and neutral is used with bright, dark, or bold colors. Do not exceed the maximum colorant load that the gelcoat can accept or it may not have the desired cure or physical properties. Never attempt to color match gelcoat on the actual project, always make a test sample on the side. In addition, always add colorants prior to adding MEKP to catalyze the gelcoat.
The gelcoat sample must be fully cured, wet sanded, then buffed to see what the final tone will look like. Perform sample tests until you achieve the proper color, then replicate that color on the actual project.
There are gelcoat brands that will sell color matched gelcoat for many of the boat manufacturers, but it should be clear that new gelcoat will not match faded gelcoat that has endured years of exposure and use. These color matched gelcoats also come at a premium price.