The newer BMW convertibles have an IR reflective dye in their leather and they don't get nearly as hot in the sun as they used to. I have absolutely no issues with my tobacco brown getting overly warm in direct sunlight with the top down even in middle of summer.
http://www.bmw.com/com/en/insights/t...echnology.html
Oyster is pretty, but I'd be afraid of dirt and pollen stains from being exposed.
Personally, I really, really like mineral gray. If it were my I'd do the Mineral Gray / Terra, but color is a very personal and subjective decision.
Don't worry too much about the seats getting too hot.