Colorblind Web Page Filter is described as 'Enter a URL, and this tool will render it as someone with color blindness will see it. There are a lot of available filters, each one simulating one of the many color blindness variants' and is an website in the Development category. There are more than 10 alternatives to Colorblind Web Page Filter, not only websites but also apps for a variety of platforms, including Mac, Windows, Google Chrome and Linux. The best alternative is Sim Daltonism, which is both free and Open Source. Other great sites and apps similar to Colorblind Web Page Filter are Vision (Freemium), Colorblinding (Free, Open Source), Coblis (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.