Studies indicate dichromats detect large, long duration spectral increments presented on bright white backgrounds with a blue-yellow colour opponent mechanism. Since opponent processes signal colour, we hypothesized that under these viewing conditions dichromats should perceive spectral increments as coloured at detection threshold. Psychophysical detection and colour discrimination thresholds were determined for normal and dichromatic humans. Test stimuli were 2°, 200 ms increments presented upon a white, 1000 td, spatially coincident background. As expected, normal observers were able to discriminate between white and spectral flashes at intensities near detection threshold intensities. Dichromatic observers required suprathreshold (≈0.30 log units) stimulus intensities to discriminate between the white and spectral flashes. The results do not support our hypothesis and alternative explanations for the elevated colour discrimination thresholds in dichromats are discussed. © 2003 Elsevier Science Ltd. All rights reserved.