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 colours.
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).
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-colour 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-colour 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). Ideal R, G, and B colour filters each pass approximately one third of the visible spectrum; in contrast, filters of the subtractive primaries C, M, and Y each pass two thirds of the visible spectrum. Red / Cyan, Green / Magenta, Blue / Yellow, are complementary pairs since white light minus red equals cyan, −cyan is red, and so on.
Enlargers which filter white light to adjust the colour balance of prints typically have controls labeled with calibrated CMY values. Colour 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 coloured surfaces. This kind of colour impression will always be built on the CMY scheme in the printing business, although to fine arts painters a red-blue-yellow colour mixing system is more familiar.
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 colour 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 colour monitors as well as on new colour LCD or OLED displays. The same pertains for cameras' displays and EVFs.
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 colour 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.
The first commercially successful film plates for colour photography used a layer of coloured potato starch with violet, red-orange and green starch powder mixed as a colour filter layer upon the light-sensitive photo emulsion. Natural colour images could be achieved through a reversal step in the development process, the Autochrome colour photo process of the brothers Lumière. This step reversed the plate into a colour 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 colour 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 colour of additive colour scheme CMY and light-sensitive for its complementary RGB colour to reproduce the projected negative. Development makes the CMY colours visible. Fixing bath is necessary to make the images persistent.
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 colour 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