RGB refers to the primary colours of light, Red, Green and Blue that are used in monitors, television screens, digital cameras and scanners. CMYK refers to the primary colours of pigment: Cyan, Magenta, Yellow, and Black. These are the inks used on the press in "4-color process printing", commonly referred to as "full colour printing".
The combination of RGB light creates white, while the combination of CMYK inks creates black. Therefore, it is physically impossible for the printing press to exactly reproduce colours as we see them on our monitors.
Many programs have the capability to convert the layout/images from the RGB colour space to the CMYK colour space. We request that you convert your colours from RGB to CMYK if your tools allow you to. By doing it yourself, you have maximum control over the results. You may notice a shift in colour when converting from RGB to CMYK.
If you do not like the appearance in CMYK, we recommend that you make adjustments while working in CMYK (usually lightening). Generally, you should specify CMYK colour builds that look a little lighter than you want, since the dots of ink "fatten up" on press, giving you more pigment on paper than you see on your monitor. Be especially careful to keep backgrounds light if there is black or dark coloured text over it, so that the text remains readable. Please see our design tips for more.