Light, as shown by Newton, consisted of seven colours, that together form the visible spectrum. However, out of these seven colours, just three have been defined as the "primary colours" since any other colour can be obtained by having a correct mixture of these three. These three primary colours are red, green and blue, are are also konwn as the "additive primary colours" in many cases.

Issac Newton devised a special disc, known as the Newton's colour disc, that is used to show how colours are able to mix together. When the disc is still, it consists of individual colours, marked as different sectors. But when the disc is rotated with a high angular velocity, the colours seem to mix together and give an impression of the disc being of just one colour.

Three three primary colours, as mentioned above, can give any other colour, just by existing together in a particular ratio. The immediate combinations of these three colours are cyan (produced by the mixing of blue and green), yellow (produced by the mixing of red and green) and magenta (produced by the mixing of red and blue).

These three colours are known as the "secondary colours" or "complimentary colours". In correct combination, they can give back the primariy colours. Yellow and cyan give green, cyan and magenta give blue, and magenta and yellow give red.These three colours are known as the "secondary colours" or "complimentary colours". In correct combination, they can give back the primariy colours. Yellow and cyan give green, cyan and magenta give blue, and magenta and yellow give red.

The mixing together of colours is utilised in electronic displays, like television screens. A colour television screen consists of tiny dots of red, green and blue that are present throughout the screen as sets of three. These colours are light up by an electron beam that sweeps the whole screen, in a very short time, lighting up specific colours of different sets, which then form a picture on the screen that is viwed from some distance away from the screen.

Filters are transparent surfaces that allow only light of a specific wavelength to pas through. For e.g., a red filter would absorb all the other wavelengths of the incident light and would only allo red to transmit. Filters find a lot of applications in many fields today.

The colour of a surface depends upon the specific wavelength tha it reflects. For e.g. when white light falls on fresh grass, it absorbs all the wavelengths, except green which reaches the eye of the observer. Hence the gras appears green. Surfaces like wood and skin reflect back not one colour, but different colours in different ratio, which are combined during perception by the human eye. Surfaces that reflect all three colours appear white. Those that reflect none, appear black.

The three primary colours are also used during printing/painting. Instead of printing secondary colours directly onto a surface, the surface is coated with three layers over a layer of white that combine in different places to give impression of different colours.

Pigments are colouring substances that are very frequently used by many industries today like the dyeing industry, printing industry etc. They are also used in homes, as food dyes. Food dyes are used to give the food a more intense colour than it really has. Dyes are mainly derived from organic substances, like plants and insects.