RGB, which stands for Red, Green, and Blue, is the color model used in digital video cameras and displays. The RGB model is additive, which means that different colors are created by combining these three primary colors at various intensities.
When it comes to chroma keying, or "green screening," the choice of green, blue, or occasionally red isn't directly related to the RGB color model. Instead, it's based more on practical considerations. Here's why:
1. Green is used most frequently because digital cameras are most sensitive to green, due to the Bayer filter mosaic pattern used in most digital camera sensors. This filter pattern has twice as many green sensors as red or blue to mimic the human eye's own sensitivity to green. As a result, green screens can produce the cleanest key with the least noise.
2. Blue is the second most common choice. It was more popular in the past, especially in film, because film emulsion was more sensitive to blue light. Blue screens are also used in scenes where the subject is predominantly green (like plants).
3. Red is rarely used because it's a color often present in skin tones, and using it could result in parts of a person being keyed (made transparent) along with the background.
So while the colors used in chroma keying are indeed the primary colors of the RGB model used in digital cameras, the reason those specific colors are used has more to do with how cameras (and film) respond to different colors and less to do with the RGB model per se.