Is there any downside to using a darker color (medium grey) on a deck other than it gets hot to touch??

It absorbs the sun's rays and deteriorates the finish quicker. You'll need to wax often with UV boat wax and store boat covered or indoors.

Navy vessel??

Peep my " combat zone thread" in the pantera section.
I haven't used the boat a lot so I can't give you any major cons yet. But I think it looks pretty pimp!

They look great but its like riding in a skillett. lol. Im doing blue on mine as we speak. it will be hot as well. but its gonna look cool.