Rayban sells glasses with a gradient. That is dark on top, lighter on the bottom. Got a pair about a year ago and love them. Had to order from their website at the time to get those lenses...i've seen them advertised at other sites since then. They didn't cost anymore than the regular lenses.
If you are ALPA the magazine had an article sometime back about gradients and which color to choose as a pilot. Might be able to search for it.