Interestingly enough, the less expensive helmets, which are shaped more "round" than aero and also have more surface contact area (fewer vents) have proven safer in crashes. The science behind it is that there are certain higher-end helmets that because of their shape "snag" on the road when you crash, increasing your likelihood of a neck or spinal cord injury.

http://www.helmets.org/ventsqua.htm

Emily, one reason why most road riders don't use visors is that it forces you to raise your head/hold your neck even higher. But, the market seems to want visors on road helmets which is why you see removable visors on many helmets, including some higher-end ones like the pneumo (New-Mo).

Regarding color, I don't think dark helmets are hotter, but they are definitely less visible. I personally wish helmets didn't come in dark blue or black, but that's just one of my Safety Jane-isms.

Hey, the Atmos is coming in pink this season!!! I should have mine this month.