I've noticed that on a few levels, certain hidden clues cannot really be seen unless viewed from a particular angle, due to the very subtle color difference between the text and the background. I know that this is intended, and is probably not a problem for most of us; however, I have noticed that when I try some of these levels on a slightly older monitor that I have at home, the clues are literally impossible to see. This older monitor does not have a flat screen, and is not as bright as most newer monitors. Even at the brightest setting, I cannot see some of the gray-on-black clues. Others have posted similar complaints on the forums, so I just thought that the issue belonged in this section. It is a difficult problem to address, since changing the text color would make things too easy, but not changing it means that certain users are quite literally left in the dark unless they have access to a different screen. My brain is too burnt out right now from trying to solve my current riddle to propose a decent solution, but I'm sure some the clever minds that dwell here in the forums might have an answer.
Seeing the pictures through an angle is just an alternative way to solve them. The right way is to take a screenshot and increase the luminosity with a retouching software. The clues are even darker in the advanced level.
Here is a list of useful softwares : http://forum.ouverture-facile.com/viewtopic.php?id=136