Definition of Colour

The Concise Oxford Dictionary describes color as “sensation produced on eye by rays of light when resolved as by prism into different wavelengths; one, or any mixture, of the constituents into which light can be separated as in a spectrum or rainbow.” The Collins Gem Dictionary says, “Appearance of things as a result of reflecting light.” 
The McGraw Hill Dictionary of scientific and technical terms describes color as, “general term that refers to the wavelength composition of light, with particular reference to its visual appearance.” 
Merriam Webster Dictionary defines color as,

  • a phenomenon of light (as red, brown, pink or gray or visual perception that enables one to differentiate otherwise identical objects.

  • the aspect of objects and light sources that may be described in terms of hue, lightness and saturation for objects and hue, brightness and saturation for light sources.

  • a hue as contrasted with black, white or gray.

From these definitions it is clear that for a human eye as well as its brain and mind to perceive color, light is essential. 
Sir Isaac Newton, the renowned English scientist, in the year 1622 AD, conducted an experiment in which he used a prism and directed light through it. The result was the appearance of seven colors, which he called a color spectrum.

Colour Classification

This color classification of Newton is relevant even today. The color spectrum he had arrived at is akin to the color spectrum of a rainbow. The light is radiant but a visible energy of electromagnetic wave motion, which moves through space at an incredible speed of 186,000 miles per second. Like X-rays, or, radio signals it is transmitted through electric vibrations and magnetic fields, though, of course, at different frequencies. It moves in waves and the measure for lights wavelength is Angstrom Units (AU).

Primary Colors: 
Red, Blue and Yellow are considered the primary colors because they are pure colors, which are beyond production by mixing other colors. Since we can arrive at any color by mixing these three colors in different proportions, these are rightly identified as primary colors.  

Secondary Colors: 

If we mix two primary colors in equal parts, we get the secondary colors viz., violet, green and orange. (violet = blue + red, green = blue + yellow, orange = yellow + red). 
Tertiary Colors: 
By mixing a primary color and a secondary color in equal proportion, we get tertiary colors.