Animals have evolved to change colors for a variety of reasons. Mammals will change completely from brown to white during the changing seasons. The pineal gland (a part of the brain) detects the changing season and sends hormones to gradually change colors through seasonal shedding or molting.
As they shed their fur or feathers, the undercoat starts to come in white, and may be more dense to keep the animal warm. But these animals have little to no control over this process, just like we have little to no control whether we get freckles or grey hair.
One of the most well known color changers are chameleons! Their color changes are also controlled hormonally, but at a much faster speed than the seasonal changes found in some mammals and birds. it is a myth that they change color to blend into their surroundings. Instead, they use it to communicate their emotions and even regulate their body temperature!
Being ectothermic (commonly called “cold blooded”), chameleons would have a hard time controlling their body temperature without their ability to change color. When they are too hot, they will turn a lighter color, and when they are too cold they will become darker. How they do this though is all about the layers of their skin.
Many animals have color cells in their skin known as chromatophores. One type of chromatophore are iridiphores, the cells make blue and iridescent colors. Chameleons have many tiny nanocrystals in their iridiphores, that reflect wavelengths of light from their surroundings. Chameleons have two layers of these in their skin. When the chameleon is relaxed the crystals are closer together, and reflect smaller wavelengths like blue, and when the chameleon is excited or stressed and the crystals are further apart, showing as longer wavelengths of color like red and yellows.
Many chameleons don’t have the same vibrance and range of color as the more popular panther (Furcifer pardalis) and veiled chameleons (Chamaeleo calyptratus). Still, most will show a tendency for cooler blues and greens when calm, and warmer yellows reds and dark colors when agitated.
Chameleon’s color changing abilities are controlled by their level of stress or excitement so though camouflage is a byproduct, chameleons actually change color to communicate with others and adjust their body temperature.
Another amazing family of color changing creatures are the cephalopods! These include cuttlefish, squid, octopus etc.
They are able to change colors rapidly and some can even change their shape too. Cephalopods have stretchy sacks full of pigment, connected to muscles, that allow the animal to change color with speed and precision. When the chromatophore expands, the color shows more than when it contracts.
The muscles that control the cells are connected to the nervous system, so the cephalopod has complete control of the chromatophore’s movement. This is how it can change color so fast and accurate.
Beneath the regular yellow, brown, and red chromatophores are the iridophores.
In cephalopods, the Iridophores are under a layer of chromatophores, so the amount that the chromatophore expands and contracts also determines how much of the Iridophores are shown. The movement of the Iridophores, unlike the movement of the chromatophores, are not controlled by the nervous system. Recent studies have shown that they may actually be controlled by hormones of the cephalopod.
As described before, the iridophores reflect light using crystalline plates. The orientation of the crystalline plates determines the colors it will show.
You can learn more about the amazing cephalopods here: https://claralogsdon.com/cephalopods/
Sources: Smithsonian, LiveScience, Biology for Better