Color matching pigment can be tricky in minor repairs where the surrounding gel coat has already seen some solar abuse... it's not such a factor when dealing with or recoating entire surfaces. My best friend Tommy was a master glassworker at his family's surf shop, and he'd always comment upon the difficulty of exactly matching color pigments during minor repairs, particularly on older boards which had seen some sun. Boats aren't much different, as far as solar abuse goes.
Matching gel color is by far the hardest part of a repair. What may look good on initial application, may be a horrid match after final buffing. Feathering edges doesn't apply either, unless a clear coat is a final layer. Also what matches today with any gelcoat older than 5 years old, may not next year, as different aged gelcoat will color change at different rates. Dark colors will show uneven sheen most and there are many, many shades of white. Gelcoating to an edge or seam is by far best, and the reason why auto body shops paint an entire car door, rather than trying to repair damage in the middle and blend edges. Above vids are a great starting point to be supplemented with experience.