A stimulus is described that demonstrates the spatial pooling of colour information in the visual system. Chequerboards (or gratings) consisting of alternating squares (or stripes) of complementary colours become achromatic at particular spatial scales; such stimuli have been named 'transchromatic' stimuli. Colour pools are much larger than the receptive fields that respond to luminance contrast. Some measurements are described which form the basis for estimates of the size of the colour pools. The size of colour pools varies according to the colours involved. For red-cyan and green-magenta complementary pairs colour is pooled at spatial frequencies above about 7-8 cycles deg-1, implying pools whose diameter is around 8 min arc. For yellow-blue complementary pairs the corresponding figures are about 4 cycles deg-1 and 15 min arc. Some phenomena of normal colour vision, colour blindness, and the development of infant vision are discussed in the light of these findings.