Primary Colors

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The definition of "primary color" lies in the eye of the beholder. There is nothing intrinsic in light itself that makes a color primary; they are just different frequencies or combinations of frequencies of electromagnetic waves. We might select the colors (hues, frequencies, or wavelengths) that we choose to call primary based on our perceptions which may have a cultural or other psychological bias. We might select them based on the availability of sources of color such as in pigments. Or we might consider the biological sensitivities of our eyes which varies from person to person. The point is that the choice of colors that we choose to call primary is subjective.

Most humans are trichromatic meaning that our eyes can physically differentiate between three colors bands. Our brains interpret all the other colors that we see according to the relative intensities of light sensed within these three color bands. Some species are mostly tetrachromatic meaning that their eyes can physically differentiate between four color bands, which doesn't necessarily mean that their brains can interpret any more colors than we can but typically, the fourth color is in the ultraviolet spectrum which most of us cannot see..Some species are dichromatic with eyes capable of differentiating only two color bands. Some species lack color receptors completely and can sense only intensity of light.

Any set of primary colors must be able, when combined, to produce the perception of all other colors in any given set or gamut.

Subtractive Primary Colors

The subtractive primary colors absorb some bands of colors of light more than they reflect other bands of colors of light. Subtractive colors are used in painting and printing. The colors typically used in subtractive processes are cyan, yellow, and magenta. When mixed together in equal quantities, the combination theoretically absorbs (does not reflect) any light. For practical purposes, some light is reflected so black is added. This is the CMYK (cyan, magenta, yellow, black) color model used in printing.

Historically, the primary colors used for the subtractive process of painting were red, yellow, and blue and these are widely accepted as primary colors in art and decorating.

Some colors that are possible in an additive color scheme are not possible in a subtractive color scheme because the combinations of subtractive colors always make a darker color.

Additive Primary Colors

Television screens and computer monitors traditionally represent colors using an additive process whereby the three additive primary colors of red, green, and blue are combined to create the perception of all the colors that we see. This is the RGB (red, green, blue) color model. When mixed together in equal proportions, these colors produce white (or gray) light.

Just as subtractive primary colors schemes are unable to produce certain light colors, additive primary color schemes are unable to produce certain dark colors because combinations of colors will always be lighter than the original colors. The range of colors that any particular color scheme can produce is called the gamut of the scheme.

Artists, home decorators, and others sometimes select the colors red, yellow, and blue as primary. This is the RYB color model. The RYB color model is the older of the three color models and probably based on observations of the psychological effects of colors.

Historically, early studies of color were made by artists and philosophers because the science necessary to fully understand the physics of color was not yet available. Older color theories may be lacking in accuracy while at the same time be useful structures for understanding perception.

Secondary and Tertiary Colors

Secondary colors are the colors made by mixing two primary colors. In the CMYK subtractive model, the secondary colors are red, green, and blue. In the RGB additive model, the secondary colors are cyan, magenta, and yellow. The secondary colors in the RYB subtractive model are green, purple, and orange.

Tertiary colors are made by mixing a primary color with a secondary color.

Color Wheels

RYB Color Wheel

Color wheels are made with the primary colors divided equidistant around the circle with mixtures of adjacent colors proportional to the distance to that color. Secondary colors lie half way between primary colors. Tertiary colors lie half way between adjacent primary and secondary colors. Where there are three primary colors, there are three secondary colors and six tertiary colors.

The RYB color wheel cannot be used for RGB or CMYK because the same sets of colors are different distances apart. On the RYB color wheel, red and yellow are 120° apart but on the RGB and CMYK wheels, red and yellow are only 60° apart.

The RGB color wheel will work for the CMYK model and vice-versa because the distances between colors are the same, just rotated.

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