Different cameras tint the colors differently. Different screens show colors very differently. We perceive colors differently in general. This means that to make it useful we need to be able to see it in contrast to something.
One very easy way is to always take the picture next to a standard A4/letter white copy/print paper, which usually is a standard neutral white. That way we can compare the differences between e-readers and photos.
It would be great if that could be a sort of standard when showing e-readers via photos.
