The black ink used in offset printing doesn’t produce a really dense or deep black. If larger surfaces are printed in black, it looks a bit grayish. Some people prefer to add a percentage of the other printing colors to the black to create what is called rich black.

This is often also done to avoid that black text or rectangles which partially overprint other objects appear ‘more black’ where they do. The black bar in the example below shows the problem.

How to create rich black

• There are many formula’s for rich black; A pretty popular choice consists of 40% cyan, 40% magenta, 40% yellow and of course 100% black. A lot of people leave yellow out of this mix since it doesn’t really add much to the density of the black and may cause issues with ink build-up and spread. On the other hand adding yellow does help to neutralize the hue.
• Another popular choice is 60% cyan, 40% magenta, 40% yellow and 100% black. This produces a slightly more neutral rich black. Please note that by adding up these percentages, you get 240%. This is the maximum ink coverage that most printers will not fuss about. Adding more undercolor might cause problems on press.
• Experienced operators adapt the mixture of colors to the content of the page. On pages that have a lot of cool colors like blue in them, they use a cool rich black which only adds cyan to the black (60C, 0M, 0Y 100K). On pages containing a lot of warm brown colors, a warm rich black is used which contains only magenta and yellow (0C, 60M, 30Y 100K).
• In case of doubt: ask your printer. They can recommend their preferred settings based on their experience with the paper, inks, varnishes and presses that they use.

Things to look out for

• Rich black shouldn’t be used for small objects such as type (unless it is poster size text) or thin lines. This is done to avoid difficult registration issues when printing. In general, use rich black for objects that are at least 5 millimeters thick.
• It is important to never use the ‘registration color’ for rich black. Printing flats with 100% of all colors in them is a nightmare for press operators.
• If you use the ‘Fill’ option in Photoshop and select ‘Black’, this adds 100% of all process colors to the image selection. You should never use this menu option.

• The above rules apply to offset printing. For digital presses the behavior of the press as well as the RIP driving it can be different. Check out this thread.