Mine is a light blue Giro; fit and visibility are the 2 main factors I consider, as well as the lighter color + vents to reflect the heat instead of absorb it.
Sometimes, when I see riders in all dark colors/helmet too, without anything reflective on, it's very, very hard to spot while driving. Even if it's a sunny day, I know how a rider can be almost invisible in the shade or road shadows, until a driver is almost upon them. Being a rider, I think that I'm (we all are, I'm sure) more alert to looking for and spotting other riders while I'm driving, but some riders make it near impossible, when they ride dark bikes, with dark clothes, dark helmets and no lights or reflective items, yes, even on a sunny day...so, if a lighter-colored helmet is the only thing that catches a driver's eye, then I'm going to make sure I've got it on my head!