In the very earliest days of evolutionary theory (1860s), Charles Darwin and Alfred Russel Wallace noted apparent anomalies regarding the very prominent 'warning colours' displayed by a wide range of animals - a phenomenon later named Aposematism by Edward Bagnall Poulton.

A large number of species (mammals, reptiles, fish, insects, etc. etc.) have evolved with very prominent colour displays which are evidently there to 'advertise' to predators that, as prey, they could be ^{[* see note 1 ]} unpalatable or poisonous etc.

The phenomenon still poses as-yet-unresolved questions for evolutionary theorists.

It's clear that it would have taken many generations to evolve the colour systems, and during that period the eye-catching patterns would presumably have made the prey animal more visible to predators - thus creating a strong evolutionary pressure for them to remain plain-coloured.

A further problem : as the predator animals gradually learned ^{[*see note 2 ]} to avoid certain colour displays, this would have caused evolutionary pressure across various prey species for their display colours to converge on a particular colour combination - instead of the myriad of combinations seen today. (Ref. Biological reviews of the Cambridge Philosophical Society 94(2))

Since the 1860s, many possible explanations have been put forward. But, to date, there are still no widely-agreed solutions to either of these questions.

An example hypothesis suggests that the evolving prey-animals may have gone through intermediary stages in which they could voluntarily show or hide their colour displays when potential predators were nearby. See :Science, Vol 379, Issue 6637 pp. 1136-1140)

*Notes :

[1] Many non-toxic organisms have adopted 'fake' colour schemes which mimic the truly-toxic species. See :Batesian mimicry (Wikipedia) This phenomenon, however, is easily explained. The mimic organisms could simply evolve their colour scheme after generations of predators had learned to avoid the highly coloured poisonous species.

[2] The question of how entire species 'learn', across generations, to avoid certain colour combinations, and how that learned behaviour is somehow passed-on to subsequent generations as an innate behaviour, has not yet been explained. See Instinct and Inheritanceplugin-autotooltip__plain plugin-autotooltip_bigInstinct and Inheritance

inexplicable

Complex behaviour patterns can be learned, but they can also be inherited.

Example 1 The female Common Cuckoo (Cuculus canorus) is a so-called 'brood parasite' - in that it lays its eggs in the nests of other birds. This can't be a 'learned' behaviour, since the bird has never met its parents (except in un-laid egg form) The cuckoo somehow inherits this behaviour - presumably via some as-yet-undiscovered genetic mechanism.