I agree with Fishtales. Your gel-coat is probably either faded or darkened depending on what it has been exposed to or covered with over the years. The spectrum in all likelihood won't be a perfect match.
If you have patience and an good eye for color ( Believe me I don't!) many have had success with a generic gel coat repair kit ( best for small cracks and nicks) by adding a very very small amount of brown tint to the white base. Through trial and error a good match can be achieved but you have to measure accurately, mix well, apply to a test base like an old piece of fiberglass or even painted metal, let the test area completely dry and then compare to the specific spot on the boat. Doing larger area gel-coat repairs is not something I would attempt. That stuff is an art and I am not an artist
One thing to keep in mind if you are going to try to fix small areas and you plan on polishing and or compounding the hull, make sure you do that before you color match since the color will be different after oxidation is removed.