Humans and other animals often use colour to recognise objects regardless of their context - as a measure of material properties rather than of their contrast with a background. Most work on visual communication signals is however concerned with colour differences, typically scaled by just noticeable differences (JNDs). Here we move from the prevailing physiological framework to understand what a given colour or type of colour might tell an animal about an object? To this end we consider the properties of object colour solids, which represent the colour gamut of reflective materials for a given type of animal eye. The geometry of colour solids reveals general relationships between colours and object properties which can explain why certain colours are significant to animals, and hence evolve as signals. We define a measure of colour vividness, such that points on the surface are maximally vivid and the ‘grey’ centre is minimally vivid. We show that a vivid colour for one animal is likely to vivid for others, and highly vivid colours are less easily mimicked than less vivid colours. Further, vivid colours such as black, white, red, blue and light, and certain unsaturated shades are produced by pure or orderly materials. Such materials are created and maintained against entropic processes. Vivid colours are therefore indicative of ecological affordance or biological function, so that it is valuable to have low-level psychological biases towards these colours regardless of any specific significance they might have to the reciever.