Figure 1.   People differ in their perception of hue similarity.  Most people see a yellow square at left and a reddish circle at right; they see the dots comprising each form as dissimilar in hue from the surrounding dots.   Persons with a form of colorblindness known as "protanopia" or "red-green blindness" will probably see the yellow square but not the circle;  for them, the dots in the circle are similar in hue to those in the background.