At a guess, I will take it that you know what the gel coat is and that you further know what the colour is, lets say blue. What however is blue. is it light blue, duck egg blue San Franscisco sky blue etc. etc.etc.
You will either have to go to the boat manufacturer for a colour chart and supplier information or try a distributer fro your boat. If it is an old boat you may never get an exact match. Even if you do the ravahes of Ultra Violet rays from the sun may well have faded the original colour.
that's the colour on the outside of the boat, where it came out of the mould. as opposed to the inside, which was probably spray-painted later.
if you have a cross-section of the boat, like a bit of hull cut out from fitting a vent, you can see that the gel coat colour extends a millimetre or so into the material.
Gel coat is custom matched for each application. White is not always just white. There may be some black, yellow or what ever in it to get the proper match for the repair.
Most people look at it.