Converting Colour to black and white in Photography

The colour wheel

The basis of understanding about photography begins with an understanding about the properties of light. So practically colour theory becomes the basis for understanding black-and-white photography. In Black & White photography colours can be explained as white light which comprises all other colours of the visible spectrum and black as the absence of all colours. Black-and-white film captures all portions of the visible spectrum. However sensitivity of representation of colours captured by the black & white film differs. 

Additive primaries & subtractive primaries

The primary colours of white light are red, green and blue. These are the colours that cannot be made up by adding light of other colours and which, when added together, make white light. They are therefore known as the additive primaries. 

Cyan, magenta or yellow are colours can be thought of as white light without red, white light without green and white light without blue. Based on the notion of taking away one primary colour from white light, the colours cyan, magenta and yellow are known as the subtractive primaries.

Black

Colours that lie exactly opposite across the spectrum wheel are called complementary colours. A colour filter will darken its complementary opposite, while lightening colours similar to its own. For example, skies in the blue to cyan range will be darkened by a filter from the yellow, orange or red range, and yellow-green leaves will be lightened by a similar yellow-green filter. The rule for filters with black and white is: opposite colours darken; similar colours lighten. 

It is important to understand the presence of tones as representations of colours in Black & white photography. Each tone has a salient impact on the colour representation.