Replying to: Nico
The problem is : if you have too powerful contrasts, you may have accessibility problems (dyslexic people, etc.).
So how do you provide a good experience on mobile :
if you don’t have enough contrast in poor conditions => experience might be not good.
if you have a very high contrast in good conditions => user with accessibility disease for high contrasts will get a bad experience.
As far as I know, a good way to define contrasts is to get values "a little bit higher" than recommendations (with all tools, see for example : http://www.dasplankton.de/ContrastA/ (...)