Sunday 30 March 2014

Colour - Colour by numbers

Colour Theory

If you don't know very much about colour theory, don't worry, with just three or four terms that I will explain below you will understand what I am on about here.  If not I would recommend a visit to and a read of one or two tutorials.

Colour theory is based around three primary colours, red, green and blue that, when mixed together can produce all other colours.  In terms of digital photographs the colour of an individual colour pixel can be described and measured in terms of three parameters, hue, saturation and luminance (or luminosity).  First however we must define our colour pallet.

Colour Space and Gamut

A colour space is like a massive colour pallet containing all the colours capable of being accurately displayed by digital devices.  sRGB is the default colour space of most cameras, computers and printers.  Not all devices can display all available colours however.  Printers for instance don't tend to have the same capabilities as cameras and colour monitors.  The range of colours which a particular device can reproduce is called it's colour gamut.  


This is the name given to the major colour component of the pixel.  There are 240 different hues available in the colour space sRGB. 


Saturation (or chroma) is defined as the purity of the colour.  In sRGB, again there are 240 different saturation increments between a vivid, pure colour and a totally desaturated grey.


Luminance (luminosity or brightness) is the brightness of the colour or pixel in this instance and is a reflection of the amount of incident light the colour reflects.

All image editing and digital painting packages use hue, saturation and luminance in different combinations to display all the available colours.  I particularly like the arrangement in Microsoft Paint as it maps hue and saturation in one graphic and keeps a separate bar for luminance.

Each colour carries a value for hue, saturation, luminance, red, green and blue.  Thus every colour can be identified by a unique set of numbers.

If we look at this another way.  Every coloured pixel, no matter how pale and seemingly colourless, is derived from a discrete hue.  Here are all the various shades of red that can be derived from hue number 0.  Note as hue 0 is desaturated it becomes a muddy reddish-brown before becoming grey.

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