Wilderness survival handbooks often warn readers not to eat colorful bugs. Many insects are fine survival food, apparently, but not all. The colorful ones are more likely to be unwholesome. Evidently, it is not enough for an insect or spider to be poisonous, it must also signal its toxicity to potential predators, or its poison does it no good.
Therefore, poisonous insects advertise their toxicity. Of course, there is the confusing problem of deceiving bugs. Some nonpoisonous insects are not merely colorful; they are colorful in a way that mimics a poison species. "Don't eat me, I'll make you sick," they seem to say, but they are lying.
One function of bright color in animals is to drive away predators, by serving as a warning. Insects bearing color combinations like red/black, yellow/black, orange black, or white/black are likely to be poison, or are imitating ones that are. (Incidentally, don't touch or eat insects that look hairy either, or ones with stingers or spines. In addition, some dull smooth insects are toxic as well.)
Another function of coloration is to attract sexual attention. Vivid coloration may be one signal of quality in a mate. The animal is fit, and of good heredity, appropriate colors say.
A hero in a romance needs snapping bright eyes, shiny dark hair, and strong white teeth to go with his broad shoulders and thrilling deep voice. These are signals that he is a healthy specimen who will be vital enough to produce viable offspring, and to help provide and collaborate in childrearing.
Camouflaged attractive colors
In some ways, birds, reptiles, and fish see better than humans do. They have an extra set of cones, color receptors, in their eyes, and these cones are carotenoid-enhanced compared to ours. This helps them perceive a broader range of color than humans do, and to see clearer distinctions between colors as well. This means that birds can signal each other with colors that might be invisible to mammalian predators. In fact, studies have shown that bird genders are sometimes differentiated only in ultraviolet, that flowers can attract bird visitors using ultraviolet, and that some ripe fruits attract birds with colors invisible to humans.
Plants use color to signal ripeness. In green fruit, the seed is often not fully mature, not ready to be separated from the parent plant. However, it is desirable for that fruit to be eaten when the time comes. That way, the plant's seeds will be spread, and possibly fertilized by animal manure, once they are fully ready. Therefore an apple tree, a tomato plant, or a blueberry bush signals ripeness with enticing reds, oranges, and violets when it is time to harvest.
Many people have noticed that in some animal species one gender is much more colorful than the other is. In some species, this may be so the more colorful member of a breeding pair can serve as a lure, to lead predators away from the offspring and the camouflaged and protecting parent. Perhaps breeding pairs with coloration that is equally drab or equally noticeable are both necessary to raise the offspring, or both unnecessary, as in the case of many animals that do not tend their young.
Colorful beauty has its uses. It can warn away predators, help bring the next generation into being, and speak a secret language. It can signal when the time is right for harvest, or lead the hungry eye away from food. Color is one of the most interesting and complicated adaptations of plants and animals.