While we still don’t have the tools built right into XD (and so I’m leaving this open), we do have at least one plugin to help with color blindness checking. Please do check out the plugins section of XD to see if one will work for you!Pavel Cerny commented
It can work similarly as the spellchecker. When text is on not-enough-contrasting background underline it with green wave.Pavel Cerny commented
It would be amazing to contain a possibility to validate the AA, AAA color compliance.
Or even integrate this tool http://accessible-colors.com/. This tool recommends, how I can change font color or background color to increase the contrast while maintaining the color tone. Integrate it into XD and users will immediately produce more usable designs.