A fundamental question of biology is why do organisms look the way they do? If we think about the power of natural selection to shape organisms' appearance, it is obvious that their colours and shapes have evolved to maximise their fitness. This may be through camouflage ('crypsis') or through ornamentation to attract a mate, such as peacocks.
Some of the most striking colours are to be found on toxic animals, such as brightly coloured poisonous frogs or vivid lionfish. These animals have evolved bright colours to advertise their toxicity - they warn any potential predators to stay away! We call these colour pattern warning colours or aposematic colouration. Many animals you are familiar with display warning colours, such as bees, wasps or poisonous caterpillars.
One very interesting thing about warning colours is that they can lead to mimicry . Mimicry is when one species copies the appearance of another to enhance its own fitness, either by evolving to share warning colours, or by cheating and deterring predators even without by poisonous.