I think the contrast is influented by the color of the eReader.
Have you ever tried to pur a sheet of paper around your device (better said around your display) and comparing those this way?
I got experience with black and ivory on my own and could not tell anything about a difference of the contrast. For me it sounds like an optical illusion. But just try to do the test and please report in.
P.S.: Have you ever tried a
cliplight for your 360?