We can even be a little more devious and create an image where people with normal color-vision will see one thing and people with red-green color-blindness will see another. But this is slightly more difficult and the result is only moderately satisfying. Below is an image where people with red-green color-blindness can see a circle and people with normal color-vision can see the letter "s". However, there are probably people who have mild red-green color-blindness who can see both.