Talk:Primary color/New article

This page is about actual colors. For the political book and movie, see Primary Colors

A primary colour is one of a small set of colours which is mixed with others in that set in an attempt to produce as many colours as possible.

An alternative definition is that a primary colour is one which cannot be produced by mixing others in its set.

These two definitions are not equivalent, but it can be seen that they are related. The more different a set of primaries are, the wider the range of colours that mixing them will be able to produce, but also the more difficult it will be to produce a colour similar to one of them from the others in that set.

A more strict definition can be made by requiring that either additive or subtractive colour mixture is used uniquely. It can then be said that for a given range of colours (a gamut), a colour is only primary if it cannot be made by mixing any combination of colours in the gamut.

By any of these definitions, even for a given gamut, there is no such thing as the primary colours; different sets can be and are used in different circumstances.

Number of primaries

The cone cells in the retina of the human eye contain three different light sensitive proteins which are involved in colour vision. These proteins (rhodopsin cyanopsin and iodopsin) absorb light over three different overlapping ranges of wavelength.

Because of these three sensitivities, a set of two primary colours will not be able to produce anywhere near all of the colours that a human can see, and a set of four primary colours will not be able to produce a significantly wider range of colour than one with three well-chosen primaries.

Humans and animals whose vision can be described using systems of three primary colours are known as trichromats. There are some animals, called dichromats, whose vision is best described using a system of two colours. Humans with certain forms of colour-blindness also have dichromatic vision.

Some animals, called tetrachromats, can see four primary colours. Research REF has also shown that a very few humans may have tetrachromatic vision. It is believed that this is caused by a mutation.

A system of one primary could not describe colour vision at all. It is monochromatic, like black and white television.

Mathematical description

The range of colours in normal human vision can be considered as a three dimensional space. Any colour is a vector, and a set of primary colours should form a set of basis vectors in the so-called colour space.

Additive versus Subtractive colour

Systems where different colours of lights are superimposed upon each other to produce different colours are said to use additive colour mixture. Sets of primary colours meant for use with such systems are called additive primary colours or primary colours of light. By far the most used set of additive primaries is red, green and blue.

Systems where colours are produced by allowing light to fall upon mixtures of pigments which absorb at different wavelengths use subtractive colour mixture. Sets of primary colours meant for use with these systems are called subtractive primary colours, primary colours for paint or primary pigments. One example of a set of subtractive primaries is cyan, magenta and yellow.

Black and White

The colour black is produced in additive colour mixture by using no light at all (darkness), and in subtractive mixture by using a pigment which absorbs all light.

In additive colour mixture, some maximum amount is defined for each primary. When the combination of the maximum amount of each primary has the approximate colour of daylight it is usually labelled as white.

In subtractive colour mixture, white is the name given to the colour of the perfect diffuse reflector, that is, a white pigment is one which subtracts nothing from the light which falls upon it.

Secondary and complimentary colours

An equal mix of any two from a set of three primary colours is called a secondary colour. More complicated combinations are called tertiary colours.

The secondary colour made by adding two primaries from a set of three is called the compliment of the third. For any given set of additive primary colours, the complimentary colours form a set of subtractive primary colours, and vice-versa. For example, the set of subtractive primaries cyan, magenta and yellow are usually considered to be the respective compliments of the additive primaries red, green and blue.

Common Sets of Primaries

Red, Green and Blue

Main article: RGB color model

The most commonly used set of additive primary colours called are called red green and blue. They are usually labeled $R$, $G$ and $B$. The three colours emitted by the phosphors used in making colour television screens and computer displays use this system, as do many more modern LCD and similar displays.

It is important to note that simply labelling primaries as red green and blue cannot result in accurate colour reproduction. Because of the variation in the shades of red green and blue, colour calibration is often used.

sRGB is a strictly defined system which uses primaries called red green and blue. It is in widespread use in computing.

Cyan Magenta and Yellow

Cyan magenta and yellow, ($C$, $M$, and $Y$) are a set subtractive primaries, which are often defined or considered to be compliment of the red green blue system.

This set is very widely used in printing, either on its own, or more often as a part of a Cyan magenta yellow and black system (see below).

Cyan Magenta, Yellow and black

The most common set of inks used in printing are cyan, magenta, yellow and black. These are labelled $C$, $M$, $Y$, and $K$, (or $k$) respectively.

The use of $K$ prevents since $B$ being mistaken for blue. The K may stand for the letter K in the word "black", or for "key" after the printing device key plate, which often uses black ink.

The system produces darker colours by including black ink, rather than just using more of the other primaries. Black ink is often cheaper, and the method uses less ink overall, which may also prevent the medium becoming saturated with ink.

Red, Yellow and Blue

Young children are often taught that the primary colours are red, yellow and blue. These colours form a set of subtractive primaries which is similar to one using cyan magenta and yellow, with a blue substituted for cyan, and red for magenta.

Some paints, especially those for children, are made in colours labeled red, yellow and blue. The pigments used are deliberately selected to provide a good range of colours on mixing. When these such paints are used, it is valid to say that red yellow and blue form a set of primary colours.

X Y and Z

In 1931, the International Commission on Illumination (usually abbreviated CIE for its French name) published a set of colour matching functions called X, Y and Z. These do not represent colour sensations in normal human vision on their own, however, they were carefully selected so that all of human colour vision can be represented by an additive mixture of the new XYZ primaries.

The mathematical functions which describe the XYZ primaries were based on a set of measurements taken using averages of observations of a two degree illuminated field. For this reason they are usually referred to as the 1931 CIE standard observer, or the 2° observer.

In 1964, a slightly different set of primaries, also called X Y and Z, were published. These were based on observations of a ten degree field, so they are usually subscripted $X_{10}$ $Y_{10}$ $Z_{10}$.