Adjust DHARMa plots for color blindness

[florianhartig]

I was told that the red-block color scheme in many DHARMa plots is not suitable for color blindness (my bad, didn't realise that red-black is nearly as bad as red-green).

Color-blind people, feel free to comment what the best solution would be.

For the moment, I think I would maybe move to an option to change signal colours in DHARMa from red to something else (e.g. blue)?