Colour reproduction

The Young-Helmholtz three colour theory is the basis of all colour reproduction and demonstrated by every printed or projected colour reproduction.

Originally postulated in the 19th century the theory proposed that all colours could be reproduced by mixtures of the 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 technique of three monochrome negatives using colour filters remained in use for high value images, such as historic documents, where long term preservation of the negative was paramount. Nowadays, the permanence of digital scans has rendered this redundant.

RGB / CMY

Red, Green, & Blue are the additive primary colours since when projected together at equal intensity they make white light. Cyan, Magenta, & Yellow are the subtractive 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.

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.

There are alternatives to RGB, such as CMYK - which is often used to specify colours to computer printers.

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.

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.

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.

Whereas RGB is easily translated into values for video displays, CMYK is easier to convert into values for printing.

Colour film

All colour film has at least three light sensitive layers for the different colours, some modern film has four layers 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 and any laser or inkjet colour printer must have at least three colours, almost always four including black, and sometimes as many as seven.