Sim Daltonism - From the perspective of a color blind person, some colors are impossible to distinguish.
Coblis - If you are not suffering from a color vision deficiency it is very hard to imagine how it looks...
Color.review - Colors that look and work great for everyone.
Stark - Contrast check and color blindness simulator for Sketch.
Color Oracle - Color Oracle is a colorblindness simulator for Window, Mac and Linux.
accessiBe - Making websites accessible to people with disabilities