The images below can help reveal certain kinds of color-blindness. (I am not a doctor of any kind — this is information only!) People with normal color vision will see one number in each disc below; people with certain kinds of color-blindness see a different number.
These do seem to actually work. I have a friend who is color-blind, and he reports there doesn't seem to be a clear number, but the one he comes up with is unfailingly the alternate, not the primary.
Whereas my friend doesn't really see much of anything at all in the discs, to me the numbers are unmistakeable and prominent. Overall, it gives one an appreciation of what color-blind folks are missing!
Highlight the caption below the image to reveal the possible numbers. The one on the left is the primary (which people with normal color vision see). The one on the right is the alternate.
|5 or 2?||74 or 21?|
|8 or 3?||29 or 70?|