Colors will never match unless you end at edges, etc. Color matching is a skill and I'd do the best you can starting with pure white and SLOWLY adding color pigments to tint....and hopefully be happy with your result.
This is the reason an auto body shop will repaint an entire section, door, hood etc...rather than "'fairing" in colors.
Lastly, gelcoat will change colors depending on age, UV exposure, climates, etc. What was new 5 years ago won't *precisely* match the batch made today.
Oh...and the repair guy asking for the "color combo or tint color" is displaying their [lack of] skill of working with gel repairs. However, starting with a factory supplied batch can usually be acceptable or the best point to start with color matching. Color mixing under artificial light is usually deceptive too.
I have seen a computer matched color for a Cat boat a guy at our club has. He was able to take a hatch up for a match. It was very hard to tell.
I have been told a bit a yellow and a bit of black. I tried all sorts of mixing and did get sort of close once on my sunfish. I can say I am horrible at color matching. The moral of my story... Just out of the can white is TOOOOO white and stands out. A couple of small dots of black and some yellow takes just enough of the white out to make it not so bad. Keep in mind I am mixing 2 Oz at a time for what I am patching so accurate repeatable matching is just not a thing.
If you need a perfect match you will need to hire a pro.