Stimuli varying in intensity and chromaticity, presented on numerous backgr
ounds, were classified into red/green, blue/yellow and white/black opponent
color categories. These measurements revealed the shapes of the boundaries
that separate opponent colors in three-dimensional color space. Opponent c
olor classification boundaries were generally not planar, but their shapes
could be summarized by a piecewise linear model in which increment and decr
ement color signals are combined with different weights at two stages to pr
oduce opponent color sensations. The effect of background light on classifi
cation was largely explained by separate gain changes in increment and decr
ement cone signals. (C) 1999 Published by Elsevier Science Ltd. All rights
reserved.