Pop quiz: what color is the mirror inside your camera? If you answered “No color. It’s a mirror. What the hell are you on about this time, Sorrel?” then you’re dead wrong. Kinda. It turns out that mirrors are ever-so-slightly green.
The mirror doesn’t reflect everything that falls on it, thanks to the fact that it isn’t 100 perfect. And mirrors reflect a little bit more green than they do other colors. Normally you’d never see this effect, but if you snap a picture (using a mirrory DSLR or just your iPhone) of a pair of mirrors facing each other, the infinite regress allows the greeness to be magnified every time the light is bounced back from the shiny surfaces.
The result is a green hue which may have a familiar color – it looks to me a lot like the green color you see on the edge of a sheet of glass, especially a thick plate glass window. Not surprising, as mirrors are glass, too.
Should you care? Not really. The white balance in your camera is probably off way more than the cast introduced by a mirror, and remember, the mirror inside your camera pops up out of the way when you take a photo (which is where the “reflex” in “Single Lens Reflex’ comes from). Nevertheless, it’s a fascinating phenomenon, and the video from VSauce is well worth the five minutes it takes to watch.