Secondary color

A secondary color is a color made by mixing two primary colors in a given color space.

Additive secondaries

Light (RGB)
For the human eye, the best primary colors of light are red, green, and blue. Combining the wavelengths of light we see as these colors produces the greatest range of visible color.

green + red = yellow
red + blue = magenta
blue + green = cyan

That is, the primary and secondary RGB colors (with secondary colors in boldface) are:

green, yellow, red, magenta, blue, cyan, green

Because color is defined as the wavelengths of the emitted light, combining RGB colors means adding light (thus the term “additive color”), and the combinations are brighter. When all three primaries (or for that matter all three secondaries) are combined in equal amounts, the result is white.

The RGB secondary colors produced by the addition of light turn out to be the best primary colors for pigments, the mixing of which subtracts light.

Subtractive secondaries

Pigments, such as inks and paint, display color by absorbing some wavelengths of light and reflecting the remainder. When pigments are combined, they absorb the combination of their colors, and reflect less. Thus, combining pigments results in a darker color. This is called subtractive color-mixing, as mixing pigments subtracts wavelengths from the light that is reflected.

Printing (CMYK)
The mixture of equal amounts of these colors produce the secondary colors red, blue, and “lime” green (the RGB primary colors of light), as follows:

yellow + magenta = red
magenta + cyan = blue
cyan + yellow = green

That is, the primary and secondary CMY colors (with secondary colors in boldface) are:

yellow, red, magenta, blue, cyan, green, yellow

Ideally, combining three perfect primary colors in equal amounts would produce black, but this is impossible to achieve in practice. Therefore a “key” pigment, usually black, is added to printing to produce dark shades more efficiently. This combination is referred to as CMYK, where K stands for Key.

Traditional painting (RYB)

Before the discovery of CMY, at least as far back as Goethe, the best primary colors were thought to be red, yellow, and blue. Mixing these pigments in equal amounts produces orange, green, and purple:

yellow + red = orange
red + blue = purple
blue + yellow = green

That is, the primary and secondary RYB colors (with secondary colors in boldface) are:

yellow, orange, red, purple, blue, green, yellow

Source From Wikipedia