If it is a chip, just use gel coat. Takes some getting used to the process but if you learn how, you are way ahead in repair costs.

The down side of epoxy is, as mentioned, it will yellow in sunlight, and it can also be very cranky to cover that repair with polyester gelcoat (the amines in epoxy blush out, causing the gelcoat to not to polymerize evenly, or at all.) That said, with some prep, it can be made to polymerize okay.

FYI, It is also not advisable to use epoxy on a polyester-made boat where the repair is perpetually underwater (i.e. a cruiser repair under the waterline), as it will eventually blister. This is not a problem on a beach cat, though.

http://www.michiganfiberglass.com/products.html

This outfit sells a bright white, high quality gelcoat and pigments that can be blended in to match aging gelcoat. It's a bit of an art but if you do it right, you will have a great match. Their supplies are better than stuff available from 'other retailers' (trust me on this...).