Colour reproduction
The Young–Helmholtz three colour theory is the basis of all colour reproduction and demonstrated by every printed or projected colour reproduction. It suggested that the human eye contains receptors for three different basic colors.
James Clerk Maxwell demonstrated in 1855 that all colours can be reproduced by mixtures of three primary colours: red, green, & blue. All printed colour material makes use of the theory although implementations may differ and often utilise more colours, especially black which does not reproduce well as a mixture of other colours (this is a limitation of colour pigments, not the theory).
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First colour images
The first colour images were shown using three monochrome positives of the same scene each taken using a different colour filter. These were then projected with the three complementary filters to make a colour composite. The very first colour picture was made by Thomas Sutton in 1861, conducted by Maxwell to support his three colour theory. The later three-color camera could make such images in one exposure using partially reflective ("semi-silvered") mirrors and colour filters.
Known as colour separations, the technique of three monochrome negatives exposed through colour filters remained in use for high value images, such as historic documents, where long term preservation of the negative was paramount, or as ideal photographic media for the production of printing plates for three-color printing presses. Nowadays digital scanning has rendered colour separation negatives redundant.
RGB / CMY
Red, Green, & Blue are the additive primary colours since when projected from three sources together at equal intensity onto one point they make white light. Cyan, Magenta, & Yellow are the subtractive primary colours since when filtered from white light, in theory, no light is left (black).
Red / Cyan, Green / Magenta, Blue / Yellow, are complementary pairs since −red is cyan, −cyan is red, and so on. Most commercial photographic colour printers have correction keys marked CMY and most amateur enlargers are marked RGB. To any experienced operator the difference is immaterial.
All color mixtures resulting from shining colored light sources give color impressions according to the RGB color scheme.
Color impressions achieved by mixed opaque paint which is applied to a surface doesn't result from light shining from these surfaces but from daylight or artificial light which is reflected by these colored surfaces. This kind of color impression will always be built on the CMY scheme in printing business, or on the painters' red-blue-yellow color mixing system in fine arts.
Digital RGB
RGB (Red, Green, Blue) is a common way of representing colour in a digital image. The colour of each pixel is represented by three numbers, being the amount of red, green and blue light making up that colour. These are normally 8-bit binary numbers, and so range between 0 and 255 when written in decimal (or x00 and xFF in Hexadecimal) — as seen in colour selectors in photo processing programs. This leads to the most frequently used colour depth of 24 bits.
RGB is the most popular way to represent colours in digital image files; GIF files use RGB in their indexed colour table, and formats such as JPEG and TIFF have RGB as the most favoured option. Most camera color sensor concepts use a pattern of red, green and blue filters for all pixels.
RGB is also a simple method of connecting computer monitors, where colour is represented by three separate analogue signals — and the computer hardware will often use RGB for colour representation, for example on old color monitors as well as on new color LCD or OLED displays. The same pertains for cameras' displays and EVFs.
Digital CMYK
CMYK (Cyan, Magenta, Yellow, Black) is a colour-representation scheme used in digital images. Colours are described by four numbers, representing the levels of each of the base colours present.
CMYK is the color scheme for printing. Any laser or inkjet colour printer must have at least three colours, almost always four including black, and sometimes as many as seven.
Some printers use CMY and CMYK, depending on the paper. Some omit usage of black for photo prints on inkjet photo paper.
Colour film
The first commercially successful film plates for color photography used a layer of colored potato starch with violet, red-orange and green starch powder mixed as a color filter layer upon the light-sensitive photo emulsion. Natural color images could be achieved through a reversal step in the development process, the Autochrome color photo process of the brothers Lumière. This step reversed the plate into a color slide. Without reversal the plate could be used as negative for prints or slides.
Modern colour film has at least three light sensitive layers for the three RGB colours, a principle already proposed by color photography pioneers like Louis Ducos du Hauron. Some modern film has even a fourth layer which compensates for the less than ideal pigments used. Colour negative film also has the familiar orange brown base colour which is an offset to compensate for pigment shortcomings. In addition to the colour sensitive layers the film also has at least two colour filter layers which must be neutralised during processing.
Photographic colour paper also has three layers, each for one color of additive color scheme CMY and light-sensitive for its complementary RGB color to reproduce the projected negative. Development makes the CMY colors visible. Fixing bath is necessary to make the images persistent.
Human vision
Finally the developed or printed image can be seen with the eyes. The human eyes are almost like biological cameras with the retina as kind of RGBK sensor. The K for black means that the low-light sensitive rod cells are not color receptors whilst the S cone cells detect short wave-length light (mainly blue-sensitive), the M cone cells detect medium wave-length light (mainly emerald-green-sensitive) and the L cone cells detect long wave-length light (mainly yellow-green-sensitive, but also red-sensitive). The cone cell signals are interlinked in different streams which combine L, M and S cone cell signals differently:
M - L = red-green-contrast S - (L+M) = blue-yellow contrast S + (L+M) = RGB luminance
