A well-known form of perceptual learning is category learning. Participants who practice a novel perceptual categorization task repeatedly show perceptual effects of the new categories on subsequent tests. Recently, we tested whether perceptual learning in general, and category learning specifically, could modify color perception (O ¨zgen & Davies, 2002). In one experiment, we required participants to discriminate pairs of colors within the same region (either blue or green). The two colors in a pair were displayed one after the other, separated by 500 ms. The task was to indicate whether or not the second color was identical to the first color. Our participants had to perform thousands of these judgments across several days. Their discrimination of colors improved steadily.