Primary colors (or primary colours in Commonwealth English) are sets of colors that can be mixed to make a useful range of colors. The primary colors are those which cannot be made by mixing other colors in a given color space.

For an additive color model, as in overlapping projected lights or in television and computer screens, the primary colors are red, green, and blue.

For a subtractive color model, as in mixing of pigments or dyes for printing, the CMY(K) set of primaries is often used. In this system the primary colors are cyan, magenta, and yellow.^[1] Other sets include the RYB system of red, yellow, blue, especially used by artists.^[2]

Primary colors are not a part of light itself. Instead, they come from the eye's physiological response to light (the way the eye works). For humans, three primary colors are usually used. In other words, human color vision is trichromatic.

Light is a spectrum of the wavelengths that can be seen by the human eye. It is an infinite-dimensional stimulus space.^[3] However, the human eye normally has only three types of color receptors, called cone cells. Each color receptor respond to different ranges of the color spectrum. Humans and other species with three types of color receptors are known as trichromats.^[3]

The additive primary colors are red, green, and blue. Because of the response curves of the three different color receptors the human eye, these colors are optimal in the sense that the largest range of colors — a gamut — seen by humans can be made by mixing light of these colors. Additive mixing of red and green light, makes shades of yellow. Mixing green and blue makes shades of cyan. Mixing red and blue makes shades of magenta, such as purple. Mixing equal proportions of the additive primaries results in shades of grey; when all three colors are fully saturated, the result is white. This color space is called the RGB ("red, green, blue") color space.

Media that use reflected light and colorants to make colors are using the subtractive color method of color mixing. In the printing industry, to produce varying colors, the subtractive primary colors yellow, cyan, and magenta are applied together in varying amounts. Subtractive color works best when the surface or paper is white, or close to it.

Mixing yellow and cyan produces shades of green; mixing yellow with magenta produces shades of red, and mixing magenta with cyan produces shades of blue. In theory, mixing equal amounts of all three pigments should make shades of grey, resulting in black when all three are fully saturated. In practice, they tend to make muddy brown colors. For this reason, a fourth "primary" pigment, black, is often used along with the cyan, magenta, and yellow colors.

The color space with the fourth pigment is the CMYK color space. The abbreviation stands for "Cyan, Magenta, Yellow, and Black" — K stands for "Kohle"^[source?] (German for coal) and is used to represent black^[source?] as 'B' could be confused with 'Blue'.

Mixtures of real materials like paint tend to be less precise. Brighter, or more specific colors can be made using natural pigments instead of mixing, and natural properties of pigments can interfere with the mixing. For example, mixing magenta and green in acrylic makes a dark cyan - which would not happen if the mixing process were perfectly subtractive.