Jamestown Distributors sells gelcoat for boat building or repairs in a rainbow of pre-mixed colors.
When you buy Gelcoat, some of it comes “UNWAXED” and others are “WITH WAX.” The wax forms an invisible skin over the gelcoat as it hardens, and allows it to cure properly. If you buy “UNWAXED” gelcoat, you will also need to buy and apply Poly Vinyl Alcohol or PVA over the repair to get the gelcoat to cure. I usually buy gelcoat “with wax.” If you need to go over gelcoat “with wax,” (laminating) you simply need to sand the gelcoat to remove the wax coating.
You pick one that is close to the color of your boat, then add tint to get it (close to) perfect. You can color test the gelcoat by applying it WITHOUT HARDENER to an undamaged surface of the boat. Get the tint right, then wipe off your trial spots with a rag with acetone.
Then, add hardener to the correctly tinted gelcoat and apply to the damaged hull. Let it cure and then sand it with 400, 600, and 800 grit, and you’ll never know that there is a patch.