Changes in colour appearance were studied under eight different illuminants. The most informative conditions were when sample and background were both contourless and isoluminant. Matching contour-less, isoluminant colour samples under a variable illuminant against the standard (C) revealed variations of colour constancy and brightness independent of luminance. Constancy was found worse for illuminants below the Planckian curve (I-ed, purple, violet), but rather similar under other illuminants. Brightness induction effects occurred over similar test hue ranges: 5PB-5GY for A, red, purple illuminants, and over 5GY-5RP hues for S and green illuminants, suggesting the operation of one dominant red-green opponent mechanism. Thus, the cardinal tritanopic axis bisects the Uniform Chromaticity plane at about 5GY outlining the border between these induction effects. Similar effects were evident for tritanopic violet, but not for yellow. (C) 2000 John Wiley & Sons, Inc.