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Colour is the visual perception that humans have of the light that reaches their eyes directly or after it has been reflected from an object.
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Light colour is the manifestation of superimposed light rays.
The human eye can only perceive three basic colours: red, green, and blue. However, the layering of some or all of the following rays can cause us to perceive different colours. The following image illustrates the phenomenon clearly.
In the diagram above, it is shown that in order to perceive yellow, the eye must receive a red ray and a green ray at the same time. So, a yellow light display does not necessarily emit yellow rays, but rather green rays and red rays simultaneously. However, a bulb that emits white light actually emits blue, red, and green rays at the same time. Finally, when an object or a light source appears black, it is because the eye is not receiving any light rays.
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A primary light colour is a colour formed by a single ray. These colours are red, green, and blue.
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A secondary light colour is a colour formed by two rays in equal proportions. These colours are yellow, cyan, and magenta.
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A monochromatic colour is a colour formed by a single type of radiation.
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A polychromatic colour is a colour formed by more than one radiation.
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Additive synthesis consists in combining colours (generally primary) in order to obtain a new colour.
The more light rays are added, the paler the light appears to us. If the three primary colours were added together in equal proportions, the resulting light would be white. However, if none of the base colours were present, the colour perceived by the eye would be black.
To perceive a colour, the eye must receive light rays. Different light sources such as the Sun or light bulbs can emit light rays. But how can we perceive a red apple if it does not emit light rays?
A pigment is a particle which has the property of absorbing the rays of certain colours and reflecting others or letting them pass through.
By sending a white light on the apple (formed by red, green, and blue rays), the apple will absorb the green and blue, but will reflect the red back towards the eye. As a result, the eye will perceive a red light coming from the apple, which will give the impression that the apple is red. However, the apple is not really red; it only reflects this colour. This phenomenon is called selective absorption, since the surface of the apple will only absorb certain colours.
The following illustration demonstrates the selective absorption of primary and secondary pigments upon reflection from an opaque object.
The following illustration demonstrates the selective absorption of primary and secondary pigments when light rays pass through a filter.
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A primary material colour represents pigments that absorb only one ray. These colours are yellow, cyan, and magenta.
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A secondary material colour represents pigments that absorb two rays. These pigments are red, green, and blue.
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Subtractive synthesis is the mixture of pigments of different colours. The blending of pigments causes a subtraction of light rays.
In a mixture of two pigments, each pigment continues to absorb its own light rays. The more pigments are mixed, the more light the mixture absorbs because it reflects a smaller proportion of light rays back to the eye. For instance, by mixing a yellow paint (which absorbs the blue colour of white light) and a cyan paint (which absorbs the red colour of white light), the mixture of these two pigments would absorb blue and red, resulting in only green being reflected. Therefore, the perception of the colour of the paint would be green. Thus, light rays have been subtracted, as a smaller proportion of light rays was reflected.
