Hi Martin...
I have done gelcoat on my boat a few times because of some crazing in the cockpit. Here in Florida , one simply takes a piece of the boat...example unscrew the hatch seat and take it to a fiberglass raw materials company...they have a photo process...and you really get a good match. Unfortunately, gelcoat doesn't last long in a can so get a quart or so and buy more if you need it... they will record your color.
If you need more info let me know....
Marv Kowalewski
The Happy Seal #138