We sailed the tropics in a boat with BLACK topsides and I did not notice any ah-so heat. Alas, our topsides are very low hence the area exposed to the sun is not all that great. Also, when we open the front hatch
and remove the washboards the boat becomes one drafty corridor.
The topsides are vertical, or, in some cases inverse, which seriously reduces the amt of heat induced.
The decks are a different story as they are exposed to hours of nearly head
on radiation. On the one hand, a teak deck
is an insulation per se, on the other hand, teak is relatively dark, and will heat up. I think I would go for very light cream decks and superstructure in grp. White, especially off-white is no good at all since it tends to reflect the light so much that one has to wear grade X dark shades all day long.
So, my 2 cents is, stick to your dream navy blue topsides and insulate the topsides from the inside (or build a sandwich boat from the water
level up). Go for light, but not off-white, sandwich decks, and light color canvas
BTW you may opt to design-in plenty of opening hatches that open both ways so that you can induce plenty of draft
while at anchor. Alternatively, shut everything up, and turn on the aircon.