Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic. For additive combination of colors, as in overlapping projected lights or in CRT displays, the primary colors normally used are red, green, and blue. For a subtractive combination of colors, as in mixing of pigments or dyes, such as in printing, the primaries normally used are magenta, yellow, and cyan, though the set of red, yellow, and blue is popular among artists. See RGB color model, CMYK color model, and RYB color model for more on these popular sets of primary colors. Any particular choice for a given set of primary colors is derived from the spectral sensitivity of each of the human cone photoreceptors; three colors that fall within each of the sensitivity ranges of each of the human cone cells are red, green, and blue. Other sets of colors can be used, though not all will well approximate the full range of color perception. For example, an early color photographic process, autochrome, typically used orange, green, and violet primaries. However, unless negative amounts of a color are allowed the gamut will be restricted by the choice of primaries. The combination of any two primary colors creates a secondary color. The most commonly used additive color primaries are the secondary colors of the most commonly used subtractive color primaries, and vice versa.