Ed Yong in The Atlantic:
Roses are red but violets aren’t blue. They’re mostly violet. The peacock begonia, however, is blue—and not just a boring matte shade, but a shiny metallic one. Its leaves are typically dark green in color, but if you look at them from the right angle, they take on a metallic blue sheen. “It’s like green silk, shot through with a deep royal blue,” says Heather Whitney from the University of Bristol.
And she thinks she knows why.
Similar metallic colours are common in nature—you can find it in the wings of many butterflies, the bibs of pigeons, the feathers of peacocks, and the shells of jewel beetles. These body parts get their color not from pigments but from microscopic structures that are found in evenly spaced layers. As light hits each layer, some gets reflected and the rest pass through. Because of the regular gaps between the layers, the reflected beams amplify each other to produce exceptionally strong colors—at least, from certain viewing angles.This is called iridescence.
Iridescence is less obvious among plants, but there are some stunning exceptions.