Not diminishing but negative returns

 From Envisioning Information by Edward R. Tufte.  Page 81.

In representing and communicating information, how are we to benefit from color's great dominion?  Human eyes are exquisitely sensitive to color variations: a trained colorist can distinguish among 1,000,000 colors, at least when tested under contrived conditions of pairwise comparison.  Some 20,000 colors are accessible to many viewers, with the constraints for practical applications set by the early limits of human visual memory rather than the capacity to discriminate locally among adjacent tints.  For encoding abstract information, however, more than 20 or 30 colors produce not diminishing but negative returns.