Over the years I have used black, white and metallic grey tablets for reading and never noticed or even considered there would be any difference reading on one color or another. The tablet that I currently use for reading is white, but that only because that was all that was available in that model at the time I bought it, so an easy choice
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LCD and OLED displays are of such high contrast (orders of magnitude better)compared to paper or E Ink that, for myself, I would consider it a far more important consideration to use a reading application that allows one to effectively manage contrast by means of paper tinting (I have used since PDA days an "aged paper" tint) and, if wanted, selecting or loading one's own paper texture (something I have not found needed for myself). Also, along those same lines, a reading application that allows easy adjustment of display brightness by a simple screen gesture is important to me.