Color is generally defined after mixing various data “channels”. The values for each channel usually come a filter that is placed in the optical path. Filters, only allow a certain window of the spectrum to pass (for example, the SDSS r filter only allows light from about 5500 to 7000 Angstroms). In digital monitors or common digital cameras, a different set of filters are used: Red, Green and Blue (commonly known as RGB) that are more optimized to the eye’s perception. On the other hand, when printing on paper, standard printers use the cyan, magenta, yellow and key (CMYK, key=black) color space.