This debate has come up on the forum before - studies done on cars in the sun show there is no difference in temperature inside a dark painted car vs a white one. To my knowledge, no one has done a similar study on
boats. But if there was a huge difference people in northern climates would insist on dark hulls.
A dark
hull can feel warmer on the outside, but any
insulation at all will cancel that out inside. One
boat designer told me that outside
water temperature actually has a bigger effect on inside temps in a
boat.
But if you want a cool color, you are pretty safe with a light blue or gray. I
recall those absorb a pretty negligible amount of heat over pure white.