I voted gray because of Kindle, IMO It's the color that's easier on the eyes for me.
While black looks cool, I think the color is too strong and intense, and normally don't like white too. But it all depends of the particular color or device. For example I like the black form the Nook STR because it's matte. The Sony PRS seems good in its black too. BUt as for the Boox M90 for example I think the black is just too strong, I think it would be distracting, and the white version seems to go better with the product. I think the difference is the size of the reader too.
Well but that's just my opinion