Maggie's Farm

I don't think the answer is a special molecule. Pigment molecules routinely absorb some wavelengths and reflect others, which produces most of the ordinary color we see around us in birds, flowers, and inks. What's special about some bird feathers, like those of hummingbirds, and some insect wings, and nacre, and so on, is not that they consist of special non-pigment molecules, but that they are constructed of very thin transparent layers.

Thin clear layers let most of the light through, but reflect up to 16%, averaging 8%. It's the same effect of partial reflection produced by a sheen of oil on a puddle, or a pane of glass (most visible when it's dark outside). The light color favored in the reflection (more like 16% than 0%) is a function of the thickness of the clear layer as an even multiple of a particular wavelength. Constant thickness produces the sensation of a pure color, abruptly darkening if you're looking from the wrong direction. Varying thickness, as found in an oil sheen, produces a rainbow swirl, varying with the angle of light on the layer and exact thickness of the oil at that point.

Richard Feynman's popular science book QED has a good description of this phenomenon, generally called iridescence. When you see colors that are abnormally bright and clear, almost like neon, or swirly with rainbow variations, suspect iridescence produced by very thin clear layers.