Color is a combination of the physical compilation of a light beam, the physiological processes in the human eye together with the connected nerves as a consequence of the stimulus of light and the processing of these stimulus in our brain. It is difficult to quantitatively measure the afore-mentioned process.

It is possible to see the content of a light beam with the help of a spectrophotometer.
This meter sorts the beam of light into wavelengths and measures the intensity of each wavelength.
These different wavelengths of light make the following color impressions on the human eye.
 

400 - 450  nanometer	VIOLET
450 - 500  nanometer	BLUE
500 - 560  nanometer	GREEN
560 - 590  nanometer	YELLOW
590 - 630  nanometer	ORANGE
630 - 780  nanometer	RED

The human eye is only sensitive for the part of the electromagnetic spectrum corresponding with the frequencies of about  4.0 x 10¹⁴ to 
7.5 x 10¹⁴  vibrations per second. However, it is not sensitive for all wavelengths to the same extent.
Link to: Human Eye/Sensitivity/page 05.01

The so-called cones in the retina of the human eye are most sensitive for a radiation of  550 nanometer and is not coincidental the wavelength of the maximum intensity of the sunlight and gradually less sensitive for light with smaller and larger wavelengths than 550 nanometer.

However, in practice light exist mostly of different wavelengths, where the observed color impression depends on the intensity distribution of the wavelengths.
Two light beams can give the same color impression with considerable different spectral curves.

It appeared possible to classify color three dimensionally: the lightness or brightness as light and dark, the hue or color tone as red, orange, yellow, green, blue violet, and the saturation as more or less mixed with white light.
Link to: Attributes/Hue/Lightness/Saturation/page 04. 00

It seemed possible to reproduce a color by the additive color mixing of the right quantities of the three primary colors: red, green and blue.
Also, an additive mixture of two colors can be reproduced by the sum of two trios of primary colors, necessary to be able to reproduce both colors separately.

These observation gave considerable support to the trichromatic theories of the existence of three pigments in the retina of the human eye with a sensitivity for respective red, green en blue.

Recording the eye sensitivity curve of an average observer had great practical meaning to the CIE - 1931.
These records resulted in several measurements which could be compared and objectivity was gained.

The on a trichromatic equation relying systems of color identification, such as Color Space and Color Triangle, give the possibility to describe a color with three coordinates.

The three characteristics: the hue, the lightness and the saturation have been used for the compilation of systematic color atlas, like the Munsell atlas and the DIN atlas.
Thousands of color maps are arranged in these atlases.

Color can arise in two ways: additive and subtractive.
The additive mixing is when two light beams are observed together and one can see the sum of both light intensities.
When two colors give white light when mixed, these two colors are called: Complementary Colors.

When one lets a beam of white light reflect a mixture of pigments or let the beam pass successively different colored transparent materials than the absorption curves of the reflecting or transparent materials are added and this is called the subtractive color mixing.
When two colors give no reflection or are black when mixed, these two colors are also called: Complementary Colors.

Up to now the circumstances that can affect the perception are mentioned.
In the first place a distinction has to be made between luminous and light reflecting object.