It's a tool that brings attention and understanding to how color contrast can affect different people with visual impairments.
What’s interesting about this resource is that sheds light on the many different types of visual impairment, from Protanomaly (trouble distinguishing reds) to Achromatopsia (complete colour blindness; only being able to see shades) and how your website’s colour scheme fares for each. (via paddyduke.com)