Buy the closest match from Jamestown. Also get yellow, brown and white tints. Pour 1 oz of Gelcoat into a small cup..do NOT catalyze.
Clean an area of the hull you want to match. Add tint, 1 drop at a time and put a dab onto the cleaned area of the hull. Adjust color by adding additional tint(s) 1 drop at a time until you have as close a match as possible. Have someone else look at it to see if they agree. Record how many drops of each tint you used per 1 oz of gelcoat to get your match. This is now your recipe of drops of tints/oz of gelcoat. If satisfied with your match, wipe of the area of the hull where you were applying the samples. Acetone is a good solvent at this time. I use the small Preval disposable sprayers available from many suppliers even my local big box store. Styerene is the best thinner to use when spraying with the Preval sprayers, but others have used Acetone also. Remember......if it doesn't come put right....you can sand it off and start over.