My sister sent me this: an interesting test that shows an image only visible to people with some forms of abnormal colour vision. Or at least, it’s interesting to me — as many of my friends know, I’m a protanope. And yes, what I see in the two images on the first page is pretty much what’s shown in the answers on the second page. Here’s one of the test images:
Can you see the geometric shape?
As an aside, I like that the Wikipedia page gives examples of situations where colour “blind” people’s vision is more perceptive than most people’s. Most people I’ve met aren’t aware of this, but the test above gives you an idea of something we can see that you can’t.