Xenopeltis unicolor looks unremarkable in shade: dark brown, smooth and cylindrical. In direct sunlight the same animal produces an oily wash of green, purple and gold across its entire length. The effect comes from the microscopic structure of the scale surface, which diffracts light rather than absorbing it, and is exceptionally strong even by the standards of iridescent reptiles.
The animal lives where that light does not reach. Sunbeam snakes burrow in soft soil, mud and leaf litter through the day and emerge after dark to hunt frogs, lizards, small snakes and rodents, which they subdue by constriction. Their eyes are small and the head is wedge-shaped for pushing through substrate.
This mismatch is a useful caution against reading purpose into every trait. A colour that requires bright directional light cannot easily function as a signal or camouflage for a nocturnal burrower, so the sheen is most plausibly a by-product of a scale surface built for sliding through abrasive soil. Sunbeam snakes are also anatomically distinctive, placed in their own small family, Xenopeltidae, near the base of the modern snake tree.