Sim Daltonism - From the perspective of a color blind person, some colors are impossible to distinguish.
accessiBe - Making websites accessible to people with disabilities
Coblis - If you are not suffering from a color vision deficiency it is very hard to imagine how it looks...
Page Speaker - Modern web accessibility tools for agencies & webmasters
Colorblind Web Page Filter - Enter a URL, and this tool will render it as someone with color blindness will see it.
Koa11y - Automatically detect accessibility (a11y) issues on webpages