What is the coloration of an animal that warns predators it is toxic called?

Aposematism refers to the appearance of an animal that warns predators it is toxic, distasteful or dangerous. This warning signal is associated with the unprofitability of a prey item to potential predators.

The unprofitability may consist of any defences which make the prey difficult to eat, such as toxicity, foul taste or smell, sharp spines, or aggressive nature. Aposematism always involves an advertising signal which may take the form of conspicuous animal coloration, sounds, odours or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, since both avoid potential harm.

The term was coined by Edward Bagnall Poulton for Alfred Russel Wallace's concept of warning coloration. Aposematism is exploited in Müllerian mimicry, where species with strong defences evolve to resemble one another. By mimicking similarly coloured species, the warning signal to predators is shared, causing them to learn more quickly at less of a cost to each of the species.

A genuine aposematic signal that a species actually possesses chemical or physical defences is not the only way to deter predators. In Batesian mimicry, a mimicking species resembles an aposematic model closely enough to share the protection, while many species have bluffing deimatic displays which may startle a predator long enough to enable an otherwise undefended prey to escape.