The stunning throat feathers of the male Ruby-throated Hummingbird is what earned the bird its name. When glowing a bright red, it's a sight to behold. But one turn of the head can click off the color, like switching the lights off in a room.
Why is his gorget an iridescent red at times, and at other times, just a patch of dark feathers?
You see, the little individual feathers that make up the Ruby-throated Hummingbird's gorget are not red at all. They have no pigment or coloration within them.
So why do the throat feathers (sometimes) look red? Well, the color comes from the shape and microscopic structure of the feathers and how light interacts with those microscopic structures.
I don't want to get in over my head with something I struggle to fully comprehend myself, but the easiest way for me to understand it is when I think of these particular types of feathers like a prism. A prism is clear but when light hits the prism at the right angle, it refracts color.
So depending on the lighting and the angle at which you see the hummingbird's throat, the feathers will either appear an iridescent red or a flat black.
Here's a series of pictures of the same bird on the same branch, all taken in a 2-minute time frame. It's interesting to watch the throat color turn on and off almost with each turn of his head.