I know I was told by a local gelcoat supply and repair shop that the color will darken slightly when it cures. Approx 1 shade. You need an eye for that.
But this has shown to be true based on my experiences and those of a close friend who did a lot of gelcoat repair.
My problem is I don't have a good color eye to know what pigment to mix. It's not always black or gray to get off white. For example Mainship "white" was yellow and brown.
Let me know what you come up with...maybe my 1983 white is the same as yours.