Design Basics 101: CMYK and RGB

19 Jul 2014

For casual users of design software, it is not always apparent why the colours in a printed image or photograph sometimes come out differently to those seen on a computer monitor at the design stage. More often than not, this frustrating phenomenon is due to a misuse of colour space formats. Computer screens, video editing programs and many photo editing suites display images in the Red-Green-Blue (RGB) format, whereas images are physically printed on to paper in Cyan-Magenta-Yellow-Key (CMYK) format. An understanding of the principles behind colour spaces is essential to good design work.

RGB Colour Space

RGB is present in the majority of electronic displays, including LCD monitors, digital cameras and scanners, and is used because it affords the user the widest possible range of colours. It is described as an additive colour mode in that it mixes colours (in this case red, green and blue) to create all manner of different colours and shades. White is created in RGB by using maximum amounts of all three colours, whilst the opposite effect creates black.

CMYK Colour Space

CMYK is used by printers to transfer an image from a computer screen on to a physical object, such as a billboard, sheet of paper or t-shirt, using ink. The four colours used in this format are cyan, magenta, yellow and black (known as key when discussing colour spaces). Unlike RGB colouring, CMYK is a subtractive process, meaning each new colour removes light from the process in order to create a new colour or shade. As cyan, magenta and yellow are all colours that are naturally imbued with high levels of light (luminance), it is possible to create new colours by removing light from the mix through combining the colours and adding black.


Differences Between RGB and CMYK in Practice

The range and vibrancy of colours produced in an RGB colour space is unmatchable in CMYK. Attempts to print RGB material are misguided and may cause colours to appear washed out or darker than they were on the computer screen at the design stage. Whilst an image created in a CMYK colour space can be exactly reproduced in RGB, as any colour CMYK can produce is also achievable in RGB, the same can not be said should the circumstances be reversed. To reliably print an image in RGB format it must be converted to CMYK through the use of graphics/image editing software.

Converting RGB to CMYK

For the best and speediest conversion results, it is important to know the intended end product from the outset of any design project. Images that will be incorporated into a printed design can be converted to CMYK at the outset of the project. This will ensure that any colours present at the end of the design stage will be achievable in a CMYK colour space and can therefore be printed reliably. Software such as Adobe Photoshop has tools designed to help users get to grips with issues that arise from differing colour spaces. Using the ‘Gamut Warning’ view will turn any colour in RGB that is not present in CYMK grey, with Photoshop able to automatically replace this colour with the next best option. Users can also do this manually.