Still there is no good reason (other than the marketplace ) why many models (not just the 'commuter' types) of helmets can't come in a fully reflective color scheme.
If I could buy a fully white-retroreflective Pneumo I'd buy it at the outrageous prices they already sell for.

Helmets don't really add to conspicuity from the rear, but they do from the front, especially for the more performance oriented bike set ups.

Al