Here's an excellent you-tube video on gelcoat color mixing from Boatworks Today, a great blog that covers every aspect of fiberglass repairs. It works!
It's a two part program, this is part 1.
https://www.boatworkstoday.com/videos/gelcoat-color-matching-part-1/
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Craig & Katherine Briggs 
SN 68 Sangaris
Tropic Isle Harbor, FL