A critique of the use of colour lightness in animal studies

Colour lightness has received considerable attention owing to its diverse functional aspects, such as in thermoregulation, pathogen resistance, and photoprotection. However, the theoretical basis underlying the function of colour lightness is closely related to melanin pigments. Therefore, here we discuss that neglecting other colour-producing mechanisms may bias interpretation of the results. In general, colour lightness is indiscriminately employed as a measure of melanization. Nevertheless, animals may exhibit colours that derive from several pigmentary and structural mechanisms other than melanin. Our primary argument is that colour lightness should not be used indiscriminately before knowing the colour-producing mechanism and validating correlated physiological parameters. For instance, the use of colour lightness as a proxy for thermal or photoprotection function must be validated for pterins, ommochromes, and structural colours.