Info on the 3 variations: http://www.iamcal.com/toys/colors/
Another article: http://www.eyecaresource.com/conditions/color-blindness/
Visual test: http://www.lensshopper.com/eye-disorders/color-blindness.asp#test
Color vision deficiency simulation: http://www.tsi.enst.fr/~brettel/DaltonDemo/DD08.html
Good site for general info and links: http://www.toledo-bend.com/colorblind/Ishihara.asp