Colour modes

Have you heard designers talking about 'converting' photographs or documents? When suppling images have you been asked to send them in either CMYK or RGM colour modes? Here's a quick overview of what they are talking about.

CMYK and RGB are chosen depending on your final media.

• Print publishing uses CMYK
  For documents printed on a printing press using process colours, every item in your document must be specified CMYK, because it corresponds to the CMYK process inks used in four-colour printing presses. All images and photographs must be in CMYK colour.

• E-publishing uses RGB
  For documents that will be viewed electronically, specify colours using the RGB colour model, because it corresponds to the RGB phosphors used in cathode-ray tube (CRT) video monitors. You can also use RGB colours from a Web-swatch library available in most design software; these are 'Web-safe', they will not dither (appear coarse) in major Web browsers.

RGB (Red, Green, Blue)

RGB images use three colours to reproduce up to 16.7 million colours on-screen. Each of the RGB components can use a value ranging from 0 (black) to 255 (white). Because the RGB colors combine to create white, they are also called additive colours.

When the values of all three components are equal, the result is a shade of gray. When the value of all components is 255, the result is pure white; when all components have values of 0, the result is pure black.

CMYK (Cyan, Magenta, Yellow, Black)

CMYK colours are defined by percentages of each. Combining pure cyan (C), magenta (M), and yellow (Y) pigments would result in black by absorbing, or subtracting, all colours. For this reason they are called subtractive colours.


Black (K) ink is added for better shadow density. (The letter K came into use because black is the “key” colour for registering other colours, and because the letter B also stands for blue.)