RGB vs CMYK

Red, Green, and Blue are the "additive colours " - combine red, green and blue light, and you get white light. Cyan, Magenta and Yellow are "subtractive colours" - if you print cyan, magenta and yellow inks on paper, they ought to absorb all the light shown on them. Your eye receives no reflected light from the paper, and perceives black... in theory.

An image that is in RGB mode is optimized for display on a computer monitor. In order to reproduce that very same image using ink on paper, it must be converted to the "CMYK" colour mode.

In practice, printing inks contain impurities that prevent them from absorbing light perfectly. They do a pretty good job with light colours, but when you add them all together, they produce a murky brown rather than black. In order to get decent dark colours, black ink is added in increasing proportions, as the colour gets darker and darker.

Gamut
The word "gamut" is used to describe a range of reproducible color with a given set of tools. There are some colors which a computer monitor can display which are impossible to print using the standard " CMYK" inks Most notably, certain vibrant deep blues and rich reds are "outside the gamut" of SWOP CMYK.  There are also colors which are reproducible with CMYK inks which are impossible to represent on a computer screen. Pure Cyan is particularly problematic.

Designing for CMYK printing
In order to print properly, any image files that you supply for CMYK printing must be in CMYK mode . RGB files will look good on screen, and they will even look good when printed on many of the desktop colour printers on the market today. However, they will not separate properly when made into film, and the resulting full printing job will not look the way you expect it to look.

Inexperienced graphic designers, unfamiliar with the limitations of the CMYK gamut, often provide a steady stream of RGB files, which we relentlessly convert to CMYK mode before sending for film output. Much of the time, the colour change that occurs is slight. Every once in a while, though, we get artwork whose effectiveness is severely compromised when the colour range is compressed during the transition to CMYK mode.  It is often a challenging task to explain to the designer why there is absolutely no way to get that blue using CMYK, no matter how much we want to.

Most Tiff, JPG, GIF image files (called Bitmaps) generated by a scanner or camera are likely to be 24bit RGB format.  Applications such as Adobe PhotoShop or Corel PhotoPaint have the ability to convert them.

When working in you graphic design software, you should leave your colour files in RGB mode up until you need to print separations, or until you need to know CMYK ink values, so you can match colours in another program. Don't make repeated changes between RGB and CMYK mode, using the mode menu. Every time you switch, a little clarity is lost. One switch is no problem; 20 switches makes a difference.

You may ask, why not simply switch to CMYK mode as soon as possible?

1) RGB files are 25% smaller, and are therefore 25% faster to work with and easier to store.

2) The CMYK gamut is pretty small. If you ever want to reproduce those files for a different medium (such as the web), you'll have thrown away some potentially useful information