Two thoughts on this: practical and theoritical

Practical: Light colors will stay cool -- which is good for comfort --
and depending on the resin used in the mfg of your fiberglass boat --
may be necessary. Epoxy resins begin to soften at 129degrees F -- so a
dark color on a hot day could soften the epoxy enough to show some print
through of the fiberglass. Polyester resins soften at a higher temp.

Also, the metal flake or glitter in finishes helps the mfg hide minor
imperfections in the final finish, as do lighter colors. A dark color
realy emphasises any imperfections.

Theoretical: Considering a fish's angle of vision and that fish tend to
"see" / "feel" with their lateral lines -- I think the noise the of that
18 foot Ranger with a 63 lb Thurst trolling motor will do more to spook
the fish than a gaudy paint scheme.

Course, a while back didn't some on suggest painting the bottom of the
boat to look like a school of shad?