I came across this article that made me think of the thread Enhancing glossiness, iridescence and sheen.
It provides an overview of pigmentation and structural colors that we associate with birds. While pigmentation is important for the coloration of many birds, the blue color that we find in blue birds such as Indigo Buntings are usually structural, resulting in light scattering, where the bright colors of a hummingbird’s throat are created by refraction that results in iridescence. There’s even cases where certain pigments fluoresce a bright red when exposed to ultraviolet light.
The article doesn’t go into a lot of detail, but it’s a fun read:
How Birds Make Colorful Feathers