All customer supplied artwork must be saved in CMYK ( Cyan, Magenta, Yellow and Black ) and not in RGB ( Red, Blue and Green ). The reason for this is that the full colour designs on a printing press are created by laying fine dots of CMYK on top of one another to create the full colour spectrum. If you look at a printed colour photo under a magnifying glass you will see the dots.
On the other hand a computer monitor creates there full colour images completely differently because they are light based rather than pigment. It mixes RGB ( red, blue and green ) to create its colour spectrum. Many computer programs use RGB as there default colour format because they are primarily designed for use on the web or for home printing rather than commercial print production.
The problems arise when you bring your RGB file to a commercial printing firm to produce. It is easy for your printer to convert your RGB file to CMYK however some of the colours will change. The reason for this is that RGB has a slightly different colour gamut to CMYK – some of the colours do not have a direct equivalent in CMYK and visa-versa. This means that the conversion software has to put it in the closest alternative colour available which of course can suitably change the look of your printing. Whilst this is not a problem for most people it is always best to create your design in the colours it will be printed in.