Color Theory

Additive colour

When the process of creating colour uses sources of light – such as the tiny

points of phosphor on the surface of a domestic television tube, a computer

screen or the projected light from a video projector – this is called additive

colour. The colour is created by adding together different amounts of the

additive primaries: red, green and blue (RGB). If all three of these are

present, their combined effect will be white. If none of the light sources are

available, the result will be black (the absence of light). All the other tones

and hues that the device is able to present will be created by the precise control

of the individual (and relative) brightnesses of the RGB sources, because

mixing two primary colours creates a secondary colour: red plus blue makes

magenta, red plus green makes yellow and blue plus green makes cyan.

A colour monitor reproduces colours by presenting red, green and blue

light from individual points that are so small that the eye cannot distinguish

them as separate. The blending of these points by the eye creates the illusion

of continuous colour (a similar idea to the principle of halftone described

earlier). The individual points of colour are phosphor spots on the surface of

the glass tube, which are bombarded with a stream of electrons from three

guns in the neck of the tube. The electrons excite the phosphor, which in turn

glows. The more energy that is transferred from the electron guns, the more

light the phosphors will emit: causing the phosphors to emit more or less

light, depending on the signals received and creating the various colours seen

on the screen. Thus, as the relative amounts of RGB are altered, so the whole

gamut of colours may be produced.

Additive colour works only for self-illuminating devices; it cannot work

for print because a printed page does not generate light. Print is viewed by

the light which falls on its surface, which is then reflected back by the ink

and paper. All process colour printed reproductions and most colour

photographs are based on the subtractive principle.

Subtractive colour

Generally paper is white which, and nothing can be added to white to make colour because

they are all already there. The means whereby we create the appearance

of particular colours in photography and process-colour printing is called

subtractive colour, because we start with white and subtract from that

white (all colour) the colours we do not want. Remember: with additive

we started with nothing (black) and mixed light to build the colour we

wanted. With printing, we carefully filter out of white the wavelengths we

do not want to leave behind those that we do.

The subtractive primaries are cyan (C), magenta (M) and yellow (Y).

Cyan is two thirds of white light: it is created by mixing blue and green,

while white is created by blue, green and red. So another way to describe

cyan would be to say that it is what you would be left with if you removed

the red from white.

Magenta is blue plus red, or what you would be left with if you

removed the green from white.

Yellow is green plus red, or what would be left if you removed the blue

from white.