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The first colour basic to understand is the difference between how a monitor portrays colour and the method used to transfer a file into a printed document.
On the monitor, primary colours of light (additive primaries) are generated by different degrees of electrons. The mix of red, green and blue light (RGB) is perceived by the eye as the full RGB spectrum of colour.
On paper, cyan magenta and yellow (subtractive primaries) are mixed with black (adding crispness to dark colours) to create the CMYK spectrum.
For a photograph, colour is represented by small dots of the four ink colours, which together combine to form a complete image. Via the computer CMYK is simulated on the screen and unless the monitor is calibrated to accurately match this mode, reliable colour matching may be impossible.
The RGB spectrum does not directly convert to CMYK and many colours actually have no direct match. Colours can shift dramatically from one spectrum to the other and images or key colour choices may appear significantly different from the intended effect.
Typically converting from RGB to CMYK will ‘muddy’ an image as the intensity of the additive colour on the monitor is replaced by the subtractive colour reflected back to your eye from the printed stock.
For CMYK print requirements images converted from RGB must be correctly adjusted to achieve the originally desired colour balance, or workable alternative colours selected. (See 'Image Quality' for further information)
It is important to understand that supplying a file for print in RGB will cause major colour issues if printed by an offset press and possible colour shifts if printed digitally. Created files need to be set in CMYK mode and all imported or placed content within the final document needs to be converted to CMYK, if in RGB mode.
Adobe Photoshop, however, offers a different approach for working between colour modes. Certain filters and effects will only operate in RGB mode, so it is possible to create files in RGB as the ‘working’ file and supply a ‘save as’ copy of the final file in CMYK for print purposes. RGB files in Photoshop are also smaller file sizes compared to CMYK files, which is often an advantage when working with the file and also for distribution.
As with converting from one colour mode to another, changing directly to grayscale from either RGB or CMYK without tonal adjustment is not advised. Viewable variances between objects of different colours are often lost if their relative values are similar. As grayscale is a mono (single) colour mode, the amount of information describing image content is limited, compared to a colour mode containing a wider spectrum. Detail that the eye can see through colour differences may now be effectively hidden, once converted to slightly varied tones of a single colour. (See 'Image Quality' for further information)
