Chromaticity	The aspect of colour that remains invariant when the intensity of a light stimulus changes. Chromaticity of a colour is shown in chromaticity diagrams.
Colour	A set of signals elicited by a light stimulus in the nervous system of an animal. We quantify colour as a set of photoreceptor quantum catches (the number of absorbed photons).
Colour-Matching	Colours are matched if an animal cannot discriminate between them. We assume that colour discrimination is set by noise, which includes both fluctuations of number of absorbed quanta of light and neural noise.
Lightness	Achromatic aspect of colour relative to that of illumination. Lightness can be mediated either by a signal of a single photoreceptor spectral type or by a weighted sum of several spectral types of photoreceptors. Lightness of a reflecting surface is quantified as a ratio of a photoreceptor quantum catch (or the weighted sum) corresponding to this surface to that of the ideal white reflector.
Reflectance Spectrum	The ratio of the spectrum of light reflected from a surface to that of a surface that reflects 100% of light at all wavelengths (ideal white surface) and illuminated by the light having the same spectral composition and intensity. We often use the term `spectrum’ instead of `reflectance spectrum’ where it is clear from context that the reflectance spectrum is discussed.
Saturation	Aspect of colour that indicates how different the colour is from grey. We characterise saturation as the distance of a colour locus from an achromatic point in the Receptor Noise Limited Chromatic Diagram [36]. It is important to note that it is not known if animals perceive saturation as a distinct aspect of colour.