When the channels are compared with a difference matte (green channel minus red channel, minus blue channel), you can see how the blue and red cancel each other out and reave the background black, creating a matte of the background (see Figure 2.3). This series of images is only for illustration purposes; the software used to generate a matte works much more comprehensively.
There are still situations where you'll use blue over green. If your subject is a blond
';>,¢....._,,-l.lt:L-s say, "unnaturally blond") with fine blowing hair, then to get a decent key against green, your subject's hair will go toward red. A lot of work in matching the foreground color correction to the background composite will be required. A well-lit blue screen back ground will serve better in this situation.