I've heard that alkyd enamels can yellow if NOT in sunlight, like they need some UV light on them or they'll yellow. Awhile back I called Rustoleum, and they said "no, it wouldn't". I have read about a person who used Glidden alkyd enamel in white, and it yellowed when applied indoors.

Washing machines and dryers come to mind, these are probably painted with an oil based enamel (cheap, durable) and they do not yellow although it's possible there is some blue in there that keeps the yellow away.

With respect to brightside, since boats are outside, maybe the phenomenon of yellowing is not common. My car is going to be in the garage most of it's life, so I expect mine would yellow if anything since there is some alkyd enamel in the brightside. The hatteras white I'm using is already slightly yellow, so it may be even more prone to yellowing. If it does yellow in a few years, I can always drop another $75 and paint it gray or something.

All in all, I'm not sure if it will or will not yellow, but I do know the possible yellowing is from lack of sun light, not too much..