hi sébastien, i think you'll get a lot of different answers to your question. personally i always prefer dark coloured cases because i think it makes the screen appear lighter, through an optical illusion. you can see a demonstration of that effect here :
the squares A and B are the same colour :
demonstration :
(of course, other people may argue instead that a light colour case will make the text appear darker.
)
i think it's mostly a question of personal preference. i have experience with black and red cases and neither one of them is distracting to me at all, in fact i don't notice them at all while i'm reading (even the red one !).