As an owner of a freshly painted (1 1/2 years ago) blue boat, I can say that I'm still loving the dark blue color. I haven't noticed a big difference in the temp in the cabin, but then I have an a/c unit.
There's a few things you need to be aware of with dark colors. Just like with a black car, every speck of dirt shows. The rain streaks down the hull bringing dirt with it in big stripes down the hull. The slightest bump against the old oxidized rub rail on the dock leaves a big white streak on the boat. They come off but are a PITA. Then the big one, prep time before you paint
Expect to spend 2-3 times prepping your boat for paint
than you would for white. You have to fix EVERYTHING. Nicks, scratches, dings, print through, the whole ball of wax. I'm not talking about smearing a little filler in the nicks and calling it good, I'm talking about hours, and hours, and HOURS of sanding, then sand it again. Then find the best auto body guy you can (pay him if you have to) and have him point out the flaws, then fix it again. When the body man can no longer find any flaws, then you can paint
and only find 5 or 6 bad spots.
I think the results are worth it.