Peacocks, panther chameleons, scarlet macaws, clown fish, toucans, blue-ringed octopuses, and so many more: The animal kingdom has countless denizens with extraordinarily colorful beauty. But in many cases, scientists know much more about how the animals use their colors than about how they make them. New work continues to reveal those secrets, which often depend on the fantastically precise self-assembly of minuscule features in and on the feathers, scales, hair, and skin—a fact that makes the answers intensely interesting to soft-matter physicists and engineers in the photonics industry. ...The blue end of the spectrum, however, represents a different challenge because...