4.2:1
Contrast ratio vs white
5:1
Contrast ratio vs black
Monochromacy
Lacking color vision and can only see in shades of grey
Achromatopsia
Dichromacy
Two functioning cone cells in eyes, instead of the usual three
Protanopia
Deuteranopia
Tritanopia
Trichromacy
Three functioning cone cells
Protanomaly
Deuteranomaly
Tritanomaly