Figure 1.

Colour spaces (receptor spaces, left side; see also electronic supplementary material, box) and chromatic spaces (right side) of dichromats and trichromats illustrating the relationship between the human percepts of hue, saturation and brightness and the physiological receptor axes. As dichromatic colour space only has two dimensions, hue, saturation and brightness are not independent. A tetrachromatic space (probably possessed by birds and fish) may theoretically add a fourth dimension to colour (akin to hue or saturation), but the physical interpretation and ecological significance of this are not obvious. While organisms such as butterflies and stomatopods have many more than four types of spectral receptors, it is not clear how they are used. To our knowledge, there is no evidence for ‘pentachromatic’ or higher-dimensional colour vision. Q_S, Q_M, Q_L are quantum catches of receptors sensitive to short, medium and long wavelengths, and S, M and L in chromatic spaces depict hypothetical colours eliciting signals in the respective receptor only. NP indicates the neutral point where a spectral light appears the same to the dichromat as broad-spectrum white light.