I tried too, but eventually had to have a pot of gelcoat toned (mixed with color paste) at a painters shop. He used an electronic color measure instrument to decide the color. And then added ordinary color paste in his machine, and it was then vibrated together. Just like wall paint. Toning pastes especially made for gelcoat should be available, I just could wait. For me the ordinary worked fine. The best I could get. Beware of fading on your boat causing it to not have the same color all over. At least on deck.
Jesper Milling