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?