With all due respect to Cap'n Nat, I disagree.
When refitting a previous boat, I decided to paint
dark blue, the old gel coat was getting tired, and man the blue made her look beautiful and brand new
Heat: Assuming you are of course just painting the topsides, and not the deck
itself, then I never noticed the dark blue to be a significant heat issue. The topsides are most of the time at very narrow angles to the sun so not that effective as a heat sink. The decks on the other hand, I would have to agree with Cap'n Nat...any color other than bright white is a bad idea. Even light grey gets unbearably hot on decks here in the tropics (and definitely effects the below decks temp too).
: We used a 2-part Interlux paint
(forget exact type, but a LPU I think). I was initially concerned about maintenance
issues too until I got a scratch from a nail on piling, but it easily and quickly buffed right out and you could not tell it ever happened. Amazingly tough stuff.