Wouldn't worry too much about a dark interior getting hot, but leather seats are notoriously hot in summer and cold in winter - the winter thing is why I've ordered heated seats (plus for those cool summer evenings) and (don't laugh) it's not unheard of for me to carry a couple of beach or golf towels in the summer when I'm wearing shorts to put over the seat base to stop the hot leather burning my legs. This is why I've stuck with the cloth/leather sports seats this time - I favour the seat base in cloth or alcantara with leather bolsters over a full leather seat.
I'm out in Chicago at the moment and you see many premium cars with fur seat covers for that very reason because their summers are so hot - for example over taken by a lovely Z4 (well I like them) with brown fur seat covers yesterday. Looked crap but probably does the job.
Now if we could get seats with the cooling, and ideally the massage function, like the Merc SL then we'd be talking
