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This guide explains the differences between RGB and CMYK colour gamuts, detailing why CMYK should be used for commercially printed documents and finally showing how to check colour settings in various popular software packages. Please note when ordering that we do not check documents colour set up and automatically convert all colours to CMYK, in certain cases this can cause noticeable colour variation.
Colour Gamut differences with RGB colours: the graphics are made up from Red, Green and Blue, but with CMYK the colours are made up from Cyan, Magenta, Yellow and Black.
The RGB colour spectrum is much larger than the CMYK spectrum. i.e. there are colours that can be created in RGB that are not available in CMYK. This problem is most apparent with very bright colours such as a fluorescent orange or green. Commercial presses print onto white paper using CMYK colours, in order to get the best results files should be prepared with this in mind. 
Using software such as Photoshop its possible to readjust the colour balance after conversion to more closely match the intended colour output. If using RGB elements i.e. images in the design stage it is worth converting the elements into CMYK and rebalancing the colours during the design process.
When designing any file it is important to set up and design the document in CMYK colour. This will save any problems in trying to adjust colours afterwards which can be very difficult if not impossible. Not all software is able to create files in CMYK colour mode. For example Microsoft Word and Powerpoint are only able to create documents in RGB which must be converted before printing. If you send us a native Word or Powerpoint files we will convert them into a CMYK pdf and will send a proof for checking. 
DOs and DON’Ts - Checking Your Files before sending
Finally, a couple of quick pointers to help you understanding the issue and assist in checking.
DO check proofs on screen using a colour calibrated monitor (if possible). Be aware that with uncalibrated screens colours will vary from monitor to monitor.  DO print samples using a commercial proof printer ensuring the target profile is set to Fogra 39.
DO use the Adobe Acrobat Pro Output Preview tool to check how the file will look when output to Fogra 39.
DO use printed CMYK colour swatches to check colours if unsure.
DON’T check colours against desktop printer samples as their profiles will generally try to emulate RGB colours as opposed to printing the true CMYK colours.
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