My cruising buddy had a 80 model 1 and he would mix his own to get the match.
I know he used yellow and brown pigment to get it right.
My 78 model 1 was white but not pure white. When I had to match mine I went to LBI on Groton, Ct where I bought all my fiberglass supplies. They gave me some tips to get me started but the best one was that you want uncured gelcoat to be about 1/2 shade lighter because it will darken when cured.
Hope this helps.