ke5awf said:
Yes, technanilly you can use any color, But you must remeber that when you take a color out in FCP it will take that color out of everything in the image.
Correct. And it shouldn't come as a surprise that the three best chroma key options are also the three primary colors of light (red, blue, and green). If you want you can use a purple as you key color, but then you're asking the computer to key out a mix of red and blue. That will be harder than using a "pure" primary color.
Red isn't used very often because skin tone has red in it, and green, IIRC, is used most often because there is more image information in the green channel than in red or blue.
Lethal
EDIT: Something to always remember is that the software sees colors, not shapes. So if you tell it to key out everything that's green so good bye to your green screen (good) and your actor's green eyes (bad).
Obviously you can tweak the settings to help isolate the exact color you want to key out, but I have seen people w/very blue eyes cause problems when they've been shot on a blue screen.