CMYK
CMYK stands for the four printing inks Cyan, Magenta, Yellow and Key (black). A printing machine lays these inks over one another in fine raster dots and mixes them into the full range of colours. The more ink is applied, the darker the result – this is called subtractive colour mixing.
CMYK is mandatory for printing. Screens, by contrast, use RGB (light instead of ink). Because RGB can display colours that printing inks cannot reproduce, RGB data is converted to CMYK for printing and often looks somewhat duller or darker afterwards – especially with vivid orange, green and blue.
Our tip: create your print files in CMYK from the start. That way you can already judge on screen roughly what is printable and avoid surprises.