Printed materials can only reflect light, so they use primaries which each absorb one of the RGB colours while reflecting other wavelengths; this is subtractive mixing. Cyan absorbs red, magenta absorbs green, and yellow absorbs blue. Black (which absorbs all light) is added to the set to reproduce black text cleanly and to help make deep colours without resorting to a muddy mix. Brighter, more stable colours can be achieved by using a technique called GCR (grey component replacement). Wherever the process colour mix to create a particular colour requires equal amounts of all the primaries, a percentage of pure black is added instead. This results in better contrast and colour saturation in shadows.