Aposematism

First, the selection of "red" as a releaser in courtship and social chasing indicates a secondarily evolved use of aposematic coloring. It is assumed here that aposematism in this and other red-and-black heliconiids, and their correlated development of Müllerian mimicry, occurred in evolution before the development of red as a social releaser.

Source: wiktionary

The co-occurrence of conspicuous coloration and noxious characteristics in many coastal marine species suggests that aposematism is a common predator deterrent in the marine environment.

Source: wiktionary

Detected prey can avoid an attack by way of warning signals (aposematism), for example, bright coloration as a sign of toxicity. […] Aposematisms offer opportunities for 'cheating' species which are not poisonous but show the same color pattern, so-called Batesian mimicry.

Source: wiktionary

For example, both toxicity and coloration may be energetically costly to produce and maintain, and some theoretical models have predicted that once a predator has learned that a prey is toxic, prey should reduce the amount of energy invested in producing an honest signal. This could in turn make aposematism evolutionarily stable, as opposed to a constant arms race between toxicity, coloration and predator perception or toxic resistance

Source: wiktionary