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RGB vs CMYK

Red, Green, and Blue (RGB), is the language of computer monitors, domestic scanners and domestic printers. RGB is based on "additive colours" - combine red, green and blue light, and you get white light.

Cyan, Magenta and Yellow are "subtractive colours". - Combine cyan, magenta and yellow on paper, and you get what perceives to be black. 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(K). In order to get strong rich dark colours, black(K) ink is added in increasing proportions, as the colour gets darker and darker thus commercial printing is done in CMYK.

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 domestic desktop colour printers on the market today. However, they may not colour reproduce as expected when converted to CMYK, and the resulting full printing job, colourwise, may not look exactly the way you expected it to look.

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.

RGB Gamut
The word "gamut" is used to describe a range of colour with a given set of tools. There are some colours which computer monitors can display which are impossible to print using CMYK inks. For colour critical printing, we recommend the use of an experienced print pre-press graphic designer for best results.

	RGB Blue
	CMYK approximation of RGB blue (actually, an RGB approx of the CMYK approximation of RGB blue, since you're looking at this on an RGB monitor)
There are also colours which are reproducible with CMYK inks which are impossible to represent on a computer screen. Pure Cyan is particularly problematic.
	An RGB approximation of pure Cyan. How close is it? That depends on your monitor, and how it's calibrated.

Inexperienced graphic designers, unfamiliar with the limitations of the CMYK gamut, often supply RGB files, which we must convert to CMYK mode before sending for film / printing plate output. Much of the time, the colour change that occurs is slight. Every once in a while, though, the colour change may be more pronounced during the transition to CMYK mode. It is not possible to fully explain this to an inexperienced designer without comprehensive pre-press skills. We recommend the use of an experienced print pre-press graphic designer for best results.