Colour Models

There are two different basic colour models that digital photographers need to understand. One, the RGB model, is used in the camera itself and in the computer. The other, the CMYK model, is used when you want a professional quality print.

Red: 0
Green: 0
Blue: 0
Black
Red: 255
Green: 0
Blue: 0
Red
Red: 0
Green: 255
Blue: 0
Green
Red: 0
Green: 0
Blue: 255
Blue
Red: 255
Green: 255
Blue: 0
Yellow
Red: 255
Green: 0
Blue: 255
Magenta
Red: 0
Green: 255
Blue: 255
Cyan
Red: 255
Green: 255
Blue: 255
White

Additive ColourThe RGB Colour Model

This model is additive, based on the three colours red, green and blue. It is called additive, because colours are made by adding the light from the three basic colours in different proportions. It starts with a black screen, with no light. At the opposite end is maximum light of all colours, which creates white. Different blends of the three primary colours then create all the other available colours.

Most digital systems use what is called a 24 bit colour depth. This means that 24 bits of information are available to represent the colour information. Because there are 3 colours (or channels), we have 8 bits of information to represent each individual colour. With 8 bits of information, we can code numbers from 0 to 255. You will see these values in many of the colour dialogues in Photoshop, but note that Photoshop can also work with richer modes such as 16 or 24 bits per channel.

The table on the right shows the 8 most basic combinations of colours using the RGB model. In total, the 24 bit RGB model can represent 16,777,216 different colours.

Red:          
Green:
Blue:
#000000
The web app on the left lets you make any of these colours by adjusting the sliders or by entering numerical values for each of the primary colours. Try a few values to get an idea of how the RGB colour model works. For example, try and make an orange, or varying shades of green. How many different shades of grey can be made using this colour model* (Note, the sliders will only work with an HTML5 compliant browser).

These colours form complementary pairs: red and cyan, green and magenta, blue and yellow. This is important to understand this when you are adjusting the colour balance of an image because you cannot increase the amount of red in an image without also reducing the amount of cyan.

This is the colour model used by all digital devices such as cameras, computers and colour TV screens. If you edit an image in Photoshop, this is the colour model that you will normally use.

I have also discussed this colour model in detail for web design here.

Hue, Saturation and Lightness

For some photographers it is more natural to think of colour in terms of hue, saturation and lightness (value), usually abbreviated to HSL or HSV. This is not a separate colour model to RGB, but just a different way of thinking about the same colours. Put simply, hue is a measure of the actual shade of colour being represented, saturation is a measure of how intense or strong the particular colour is, and lightness is a measure of the brightness of the colour.

Because this is really just a different perspective on the RBG colour model, I shall not discuss it further here. The interested reader is referred to the links given below.

CMYK ColourThe CMYK Colour Model

The CMYK model is subtractive and is based on mixing coloured pigments together. Because of this, it is the model most familiar to painters and printers. It is also the colour model used by printers and, if you open up a typical inkjet printer, you will see four colour cartridges: cyan, magenta, yellow and black. These correspond to the C, M, Y and K in the CMYK colour model. What does the K stand for? "Key" - the base colour, or blacK.

In contrast with the RGB model, this starts with a blank "paper" which is white. An equal blend of all three colours then produces black, whilst different amounts of the three colours produces produce the other available colours. Black is added as a fourth colour because typically the black made from mixing the three coloured inks, so-called "composite black", is often not as black as required, so a specific extra black ink is added. Of course, this also means that printers printing a lot of documents do not use up all of their coloured inks just printing black text.

Getting a full range of colours from a CMYK colour model can be quite difficult and printers sometimes use a process known as halftoning where the printing density of each colour is varied. In pactice, printers often employ a wider range of ink colours, often as many as nine separate inks, for making high quality prints with a good dynamic range.

Converting from one colour model to the other is a complex calculation that needs detailed information about colour properties of the inks being used, the luminance of the display and so forth (the colour space). In practice a system of colour management is used to ensure that all the devices in the processing chain are clearly defined. This requires each device to be correctly calibrated using special software and equipment.

Links

You can find additional information about colour models at the following pages:

Back...back to the colour theory index