Stark is described as 'Contrast check and color blindness simulator for Sketch'. There are more than 10 alternatives to Stark for a variety of platforms, including Mac, Windows, Online / Web-based, Google Chrome and Linux. The best alternative is Sim Daltonism, which is both free and Open Source. Other great apps like Stark are Vision (Freemium), Colorblinding (Free, Open Source), Colorblind Web Page Filter (Free) and Colour Blindness Simulator (Free).
If you are not suffering from a color vision deficiency it is very hard to imagine how it looks like to be colorblind. The Color BLIndness Simulator can close this gap for you. Just play around with the sample picture or upload your own images.