RGB vs. CMYK Colour Gamuts

The range of colour that can be perceived (e.g. by the human eye) or displayed (e.g. by a computer monitor or printing press) is a colour gamut. Different devices have different colour gamuts. That is, they can produce a range of colours that vary depending on the device. That is why a colour image may look very different when viewed on different monitors, and why, although both Macintosh and Windows operating systems recognise millions of colours, only 216 of those colours are considered "safe" for web use - they are the colours that can be consistently reproduced on both Mac and Windows-based computers. To make things even more complicated, colours are created and described differently depending on how they are displayed. Computer monitors, which display in RGB, are also described as using an additive colour model (add colours together to create white), while printed materials, using CMYK to create colour, can be described as using a subtractive colour model (subtract colours to get white).

RGB Colour Space
RGB stands for Red, Green, Blue. Computer monitors, video cameras, television sets, etc. create colour using the RGB model. Under this model, a device will add varying amounts of red, green and blue light to a black background. On a colour monitor this translates into having three possible light sources for each pixel on screen. To get white, 100% of each colour is transmitted. All other colours are created by varying the amount of red, green and blue displayed. RGB is considered an additive colour model. When there are no colours, the result should be black.

CMYK Colour Space
CMYK stands for Cyan, Magenta, Yellow, and Black. Four-color printed materials are created by combining various amounts of cyan, magenta and yellow inks. Theoretically, 100% of each of those colours should create a true black, but usually the result is a muddy, dark brown colour, so black is added as a separate colour To get white (paper), no colours are added. CMYK is therefore considered a subtractive colour model.

What's it Mean?
Because of the difference in how monitors and printed materials create colour, there are often surprising differences between what is created on screen and what the output actually looks like. Some computer programs (such as Photoshop) have gamut warning capabilities, to let the user know if they have created a colour on screen that can not be accurately reproduced in print. There are also several monitor and printer calibration devices and software available. Currently the most accurate way to produce colour is to refer to known benchmarks such as the Pantone Colour System. Systems like this ensure that the designer and the printer have a common reference, regardless of how something looks on screen. When working with photographs, get accurate proofs (Kodak approval, matchprint, or similar) before the job goes on press.