"... their bold patterns and vivid coloring are as effective as a neon sign in warning their natural predators of their lethal potential. For outsides such as the men of the expedition, however, such signals meant little or nothing, and merely ensured that the only creatures they could see as they pushed through the forest were likely to be especially dangerous or even lethal"
- Candice Millard, The River of Doubt: Theodore Roosevelt's Darkest Journey
Conversely, it may also be indicative of an animal which is not actually venomous, but is trying to resemble a venomous species so that it will be left alone. One of the classic examples is that of the (venomous) coral snake and the (non-venomous) milk snakes and king snakes. Both are covered with bands of black, yellow, and red but (in North America, at least), the order of the bands varies between the two species, hence the endless variations of the following rhyme:
"Red to yellow, kills a fellow
Red to black, venom lack"
Bold warning colors aren't limited to snakes - a lot of animals have them. Even, as a non-toxic example, the skunk, with it's glaringly obvious white stripe standing out against black fur, making it on of the most conspicuous animals of the North American forests. But of all of the animals in zoos and aquariums that display warning coloration, few if any are brighter, bolder, or more beautiful than the poison dart frogs.
The dart frogs aren't the only poisonous frogs with bright colors - the mantellas of Madagascar come to mind - but they are certainly the most famous. The common name of "poison dart frog" or "poison arrow frog" (zookeepers usually say "PDF"... which can confuse the IT people to no end) conveys images of South American Indians (all species of poison dart from are from the tropical forests of the New World) rubbing a dart or arrowhead against the back of a brightly-colored amphibian, then sending the projectile - via blowgun or bow - to an unwary victim, who will then succumb to the poison. It's kinda true. I say "kinda" because, of the 170 or so species, only a tiny handful are ever confirmed as having been used for this purpose. Many of the other species aren't even that toxic, or at least to humans. Still, the fact that any of them are used for this purpose is enough to make the entire group seem pretty awesome... which they are. It's hard not to respect a thumb-sized frog that carries enough poison in its skin to kill 15 adult humans.
A scene from the movie Apocalypto, where the protagonist uses a frog (not a poison dart frog) to make blowgun darts. Not sure how accurate this really is, but hey, it's a cool visual
Even without an awesome backstory, poison dart frogs are favorites among zookeepers and hobbyists alike. They've been bred in captivity since the 1970's in Europe and America, first in zoos, gradually appearing in the private sector. Their numbers include some of the most beautiful of all frogs, as described by their evocative names, like "strawberry", "bumblebee", and the far less original "green and black." Also, because they are toxic, and therefore largely protected from predators, they tend to be very bold little frogs. No hiding under a damp leaf all day for these guys (who tend to be among the more diurnal of frogs) - they are often seen sitting in a prominent location. Or doing something - they can be surprisingly active, with males often wrestling for females. Different species tend to be compatible, leading to some beautiful mixed-species exhibits, with brilliant frogs of different colors shining from dense green vegetation.
Another neat thing about dart frog poison is where it comes from. Whereas snakes are born with venom glands (and a baby rattlesnake or cobra can pack a wallop the moment it is born or hatched), dart frogs are believed to get their poison (whole or in part) from the ants that they eat in the wild. Now, most zookeepers aren't feeding their dart frogs toxic Amazonian ants, but fruit flies or crickets, so captive-bred frogs tend not to be poisonous. Kind of like making venomoid snakes, only minus the surgery and unpleasantness and welfare issues. While keepers tend to handle PDFs gently and with gloves, they pose little health or safety risk to their caretakers when on a captive diet.
None of which is an invitation to stick one in your mouth, of course.