Chromakey Green is a pure green, hon. Chromakey Blue is a pure Blue. in RGB values (that red, green, and blue), the green is 0 Red, 255 Green, 0 blue. The Blue is 0 Red, 0 Green, 255 Blue. These colors are used becuase they very, very rarely appear in most other things, allowing you to sort of "trim out" the parts that are green, leaving only the end result. This is used in movies all the time for special effects.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)