About 10% of the population - mostly males - has a form of colorblindness.
Color Oracle is a free color blindness simulator for Windows, Mac OS and
Linux. It shows in real time what people with common color vision impairments
see.
A classical tool, made by Google in 2014, to help you to build accessible
color palette for your user interfaces and measure the accessibility of
your own color combinations.
Find, test & explore accessible colors. A pleasant and intuitive tool to
help designers to build color palettes. Certainly the tool I use the most when
I'm doing visual design.