The ability to change colors is one of the most useful adaptations in the animal kingdom. Color can camouflage, hiding you from predator and prey alike. Color can communicate, signaling to potential mates that you're open for business. Color changes can occur swiftly, or they can change with the seasons. Different situations call for different color-changing skills and nature has selected for these traits as necessary across an impressive array of species.
Different animals do this in different ways. Cephalopods, for example, depend on chromatophores — pigment containing cells that can change colors on a millisecond-to-millisecond basis, typically in response to the contraction and relaxation of muscles surrounding the cell. Chameleons achieve a similar effect through rapid molecular signaling within and between cells. Meanwhile, animals at extreme latitudes often change colors with the seasons, as colder temperatures and shorter days trigger hormonal changes that give rise to dense, white coats. Take the stoat, for example.
Stoat (Mustela erminea) aka Ermine
Closely related to weasels, stoats, like snowshoe hare, adopt a predominantly white coat in the winter months (save for their characteristically black-tipped tails). This cold-weather coat is also more densely packed, and softer than the brown fur worn the rest of the year. Perhaps not surprisingly, soft, fluffy and white are all popular features in the fur industry, where stoat coats, harvested by trappers, are referred to as ermine.
NB: Stoats look every bit as conniving in a white coat as they do in a brown one.
Snowshoe Hare (Lepus americanus)
Snowshoe hare are the perfect example of a prey species that relies on camouflage to keep from being eaten. Unlike the stoat, which ecologists suspect may rely on camouflage to hide from prey and predators, snowshoe hare depend on their coat primarily to avoid lynx, coyotes, fox and birds, as well as for warmth.
Ptarmigan (Lagopus muta)
Like the stoat and the snowshoe hare, the ptarmigan is commonly found in northern latitudes, and therefore exhibits seasonal camouflage. By now you've recognized the pattern common to these northern species: come winter, the ptarmigan will trade its "normal" plumage for a set of uniform, downy feathers. Side note: is it me, or do all these species look noticeably more adorable in their winter white? Not saying the Ptarmigan's mottled plumage isn't cool-looking or functional; on the contrary, the speckled feathers of ptarmigan do an admirable job of blending in with the rocky mountainsides where the bird tends to hang out.
But not all color-changing animals do so seasonally. Many species of amphibians, reptiles, fish and insects can change their colors relatively quickly, and reversibly. This is due mostly to the fact that they don't need to grow an entirely new coat, or set of feathers, to undergo a dramatic shift in color. Take the goldenrod crab spider, for instance, or Peron's tree frog:
Goldenrod Crab Spider (Misumena vatia)
Goldenrod Crab Spiders, as their name suggests, are often found stalking prey along the petals of intensely yellow goldenrod flowers. When hunting against such a vibrant backdrop, it pays to blend in with one's surroundings, what ecologists refer to as "cryptic mimicry.: Consequently, goldenrod spiders have evolved the capacity to change colors reversibly from white to yellow. The change from white to yellow takes about two to three weeks, and is thought to be triggered by visual feedback, while changing back takes a little under a week. Interestingly, while a few chemical precursors important to the color change have been identified (namely 3-hydroxykynurenine), much of the biochemistry responsible for cryptic mimicry in crab spiders remains a mystery.
Peron's Tree Frog (Litoria peronii)
Peron's tree frog is native to Australia, where it's also known as the Laughing Tree Frog or the Maniacal Cackle Frog. Because... well, because of this:
It's also capable of undergoing a variety of color changes with incredible speed — shifting from a pale, greenish grey color to a reddish brown with green flecks, to almost completely white, with black and yellow markings on its thighs — as its surroundings, the temperature, and the time of day dictate.
There are plenty of other examples, of course. Wikipedia's got a pretty extensive (if incomplete) page dedicated to color-changing animals. Conspicuous in their absence are the arctic fox and various species of chameleons (seriously).