Digital output devices - your monitor, scanner, digital cameral etc. create colours using combinations of just three colours - Red, Green and blue (RGB).
The printing process uses four different colours to print these images - Cyan (lightish blue), Magenta (pinky red), Yellow and BlacK (or CMYK - also known as Process colour).
Converting your work from RGB to CMYK can be done in a program such as Photoshop and this should be done before sending the work for printing. If this is not done, the printers process may apply a standard profile RGB to CMYK conversion which may leave the colours looking washed out.
Traditional printers often use Pantone® Spot colours when printing work. Spot colours are mixed like paint and printed one at a time. If you are using Spot colours in your work, these should also be converted to their CMYK equivalent before being sent to the printers. If you don't convert spot colours to process, then an extra separation printing plate may have to be produced. This means that these objects created in a spot colour may not appear on the finished job and will incur unnecessary costs.
You can check your document by printing 'separations'. In Photoshop, this can be achieved by going to IMAGE...Mode...CMYK Color. Now, when you go to the Channels palette, you will have a channel each showing just the amount of Cyan, Magenta, Yellow and Black together with a composite channel.
You can check your work by printing 'separations' on your desktop printer:
To print separations from Photoshop:
- Choose FILE...Print with Preview.
- Select Show More Options, and choose Color Management from the pop-up menu.
- Choose Separations from the Profile pop-up menu.
Note: Depending on the designated printer and print drivers on your computer, these options may also appear in the Print dialog box.
- Click Print.
Separations are printed for each of the colors in the image. After following this process, anything other than a cyan, magenta, yellow and black separation prints, then you've got unwanted colours that you will need to convert. This is also a good way of checking knockout/overprint settings.
Some RGB and Spot colours do not have a direct CMYK equivalent (this is known as "out of gamut". If you have chosen a colour that is out of gamut, Photoshop will choose the closest equivalent CMYK colour, which may be very different from the colour you intended. Unfortunately, this is something everyone has to put up with.
Getting the best from CMYK
You will get more accurate reproduction of your colours if they are made up of just one or two colours (i.e. Cyan and Magenta etc.). When using lighter shades, avoid tints that contain less than 10% of either Cyan, Magenta, Yellow or Black as they usually print much lighter than they appear on the screen. For best results, use tints containing between 10% and 30% if possible.
Finally, try to avoid large areas of the same colour as this is where colour variation can become noticeable. Here it is best to use textures or images instead. Also vignettes or gradients are best avoided as they can show up 'banding'.