Excerpts from "Theory of Color" (http://www.abelard.org/colour/col-hi.htm)

Seeing colour

The human eye has four types of light receptors: the rods which are sensitive only to black, white and shades of grey, and the cones of which there are three types, each of which responds to a different range of colour.

The rod receptors are less precise than the cone receptors in their information-collecting ability for small changes, for example in edge detection, and they respond only to light and dark. However, rods have the quality of responding to a very much lower light level than cones and are the only receptors which function at low light levels.

Should you move from a lighted area to the light level at which the cones do not function, you will be blinded temporarily. The rods will start adapting to the lower levels of light available within a few minutes. You will develop a considerable amount of vision, because of the rod's low-light response ability. Most of this adaptation occurs within five to seven minutes, but it can continue for up to an hour. At these levels of light you will no longer be able to see colours (because the cones are not working) and, because of the lower precision of the rods relative to the cones, edges will be markedly less clear or precise.

The other three types of receptors are called cones. At low light levels, cones cease to function. Cones respond to different wavelengths of light, as follows: 'red', 'green', and 'blue-violet'.

graph showing cone reponses to light - three curves

nm=nanometre; 1m=10⁹nm

This diagram shows that there is a degree of overlap between the response of the types of cones.

Also, you can see from the diagram, the blue-violet cones are less sensitive to light. Approximately twice as much light (quanta) is required to obtain from the blue-violet cones a perceived light level that is similar to that obtained from the red and green cones. Or, in other words, full response of the blue-violet cones requires more light energy than for the red or green cones. Thus, at a given light level blue-violet appears darker than red or green. This relative darkness applies also to mixes involving the various cones (colours), hence the natural brightness of yellow which stimulates the two most reactive sets of cones in the eye.

Another interesting factor is that the eye has a hard job focusing all three colours at the same time. Focusing is particularly difficult with blue-violet and this results in the haze effect caused by blue-violet light which can lead to headaches. These headaches can be relieved by using yellow-lensed sunglasses that filter out the blue-violet light.
The eye is generally more sensitive in the mid-ranges around 520nm, with its sensitivity tailing off in both directions.

Further study of the cone response diagram will show that there is some overlap of cone response. This is especially worth noting in the blue-violet area where the red cones fire in certain wavelengths. Thus we see (perceive) redness in that area of the blue-violet region and we see true blues as if they have some red added, these colours being commonly called violets. As this effect drops off, it is possible that you may see a magenta effect on both sides of a more 'bluish' area of the colour wheel.

Red, green and blue-violet are regarded as the three primary colours of light. They stimulate one cone type and the brain translates this information received by the eye into what we call colour. When two sets of cones are fired, we respond that we see for instance yellow - a mixture of red and green light. The primary colours can be seen in the spectrum or rainbow, red at one end, green in the middle and blue-violet at the other end. In between these colours may be seen secondary colours that are, perceptually, each a mixture of two of the primaries.

Thus, you will see yellow between red and green, and cyan blue between green and blue-violet. The third secondary colour of light is magenta. This is not part of the spectrum, it has no single wavelength of light, but the sensation of magenta may be perceived by looking at a combination of red and blue-violet light. (It can be distinguished between the two parts of a double rainbow). It is interesting to notice that, with the wavelengths yellow and cyan our eyes decompose the light into responses made by two cones, then our brains recombine them into the sensation of yellow or cyan. That is, we never directly perceive yellow or cyan.

A rainbow may be seen by viewing light through a simple prism. Isaac Newton named seven colours for his spectrum - red, orange, yellow, green, blue, indigo, violet. One does not really see indigo as a separate colour, and orange is a bit doubtful. Newton came from a culture where specific numbers were regarded as having mystical significance, so he added the names orange and indigo to make the magic number seven.

The secondary colours (yellow, cyan and magenta) appear brighter because two sets of cones are firing together. These secondary colours are the basic colours ofÊ colour mixing for painting. However, our children are taught crudely that the mixing colours are red, yellow and blue. This leads to much confusion later if they become interested in colour work.

To add to the confusion, different people are taught to match particular colour-names to different colours. This is particularly true of the blue area. It is sensible to exercise care when teaching these mixing colours.

Colour printing does not rely on colour mixing, but on very small dots which are so close to each other that the eye sees them as a continuous colour. The dots can be seen through a small magnifying glass.

Tertiary colours are also shown in the picture. These colours are a mixture of a primary and a secondary colour. Thus, red (a primary) and yellow (a secondary), when combined, are seen as orange.

Conversely, it becomes clear that a green effect can be obtained either by mixing yellow and cyan pigments or from a single green pigment and in similar mode for reds. Of course, all magenta pigments are mixes of two pigments.

Some other comments and interesting 'facts'

Two special states of visual response we perceive as black and as white. Black is, in fact, a total absence of light, whereas white is an effective complete over-loading of the visual system where all receptors fire at once and so distinctions cannot be made any more than in blackness. Such a state is rather akin to the state referred to as 'noise' (white noise!) in various contexts.
Some colours such as yellow form a single wavelength in the spectrum. However, when we look at yellow, two receptors of the eye are fired - red and green. The signals from these cones are recombined in the brain to give us the experience of yellow. Therefore, it is not possible for our perceptual system to distinguish whether we are looking at yellow formed by a single wavelength or a judicious mixture of red and green wavelengths.
The magenta area (the colours between red and blue-violet) has no such single wavelength in nature and so is always mixtures of wavelengths. Mixed wavelength and single wavelength colours may be distinguished by a selective use of filters. For example, viewing magenta through a yellow filter will block blue-violet light and then you will be able to see clearly the red component; that is, the magenta will appear red. Likewise, viewing the magenta through a cyan filter will block the red component leaving the blue-violet component visible.
A particularly interesting part of the spectrum is in the region of blue-violet where, strangely, the red cones also fire, making the human 'see' red in the short-wave (blue) visual light. The amount of red perceived and just how far into the short-wave light the individual sees will vary from person to person because of natural human variation.
It is even possible that this red effect increases then decreases towards the extremes of perception leaving a reddened hill with blue-violet either side. As the red component increases when moving into the magentas, the effect of this edge or wrap-over area of light seems to allow an extra number of quite clearly distinguished and subtle colours in this special region of the colour circle. Also refer to the cone response diagram.