Epicrates cenchria carries an ordinary brown-and-black pattern made of pigment. Layered over it is something different: in strong light the skin throws off bands of violet, green and gold that move as the snake moves.
The effect is structural colour. The transparent outer layer of each scale carries arrays of fine parallel ridges spaced at a scale comparable to the wavelengths of visible light. Incoming light is diffracted and interferes with itself, so different wavelengths leave at different angles. Because nothing is absorbing or emitting colour, the sheen has no fixed hue; rotate the snake and the bands shift. The same physics explains the colours of a compact disc, a soap film and a beetle's elytra.
The iridescence is brightest just after a shed, when the outer scale surface is undamaged, and dulls as the surface abrades. It is often described as a camouflage or signalling adaptation, but for a nocturnal snake there is no good evidence of a function, and it may simply be a by-product of scale microstructure.