I have consciously avoided the the term “chroma key” as historically the term applied only to video systems only. That’s not the case anymore. In rudimentary video mixers, a keyer was a mathematical process that would make a range of colors in a video signal and make it transparent. This is, of course, a common effect that television newsrooms all over would use weather map special effects.
Blue as a screen color was still predominate but green started to take over as films began getting the digital post production treatment in the late nineties. Why Green? Basically Green was easier and cheaper to light than blue, green registers brighter on electronic displays, worked well for outdoor keys (where the blue screen might match the sky) and the bold green color was less common in costumes than blue is.
And now as digital camera are replacing film, many digital sensors use a Bayer Pattern which have twice the number of green photosites than red or blue to capture luminance. This makes modern digital cameras much more sensitive to the green part of the spectrum making pulling a matte from greenscreen a little easier. Blue is still commonly used as are other colors depending on the needs of the shot.
So now with advanced software and motion controlled cameras, Chroma Key, a term that has grown now to encapsulate much more than it’s original video technique, can be used to insert backgrounds and set extensions in ways that Georges Milies and Norman Dawn could only dream of.
There are cynics today that believe modern film is too reliant on CGI and that we should return to a simpler form of real filmmaking. But as I hope you learned, that era never existed - filmmakers from the very beginning have sought to push the medium with special effects.
The undeniable truth about filmmaking is the only thing that matters is what’s on that screen. From Edwin S. Porter’s matted train station window to the modern action spectacle, it’s all about creating a window onto another world. A world where each of us can find our dreams our fears and ourselves. All these effects we have are just tools to help us get there.. And we have some fantastic tools, so use them, and make something great.