Rich black, in printing, is an ink mixture of solid black over one or more of the other (CMYK) colors, resulting in a darker tone than black ink alone generates in a printing process.
A typical rich black mixture might be 100% black ink and 50% of each of the other three inks. The colored ink under the black ink makes a "richer" result the additional inks absorb more light, resulting in a closer approximation of true black. While in theory an even richer black can be made by using 100% of each of the four inks, in practice the amount of non-black ink added is limited by the wetness that the paper and printing process can handle. Wetness is not a problem with laser printers, however, and "400% black" (as it is also known) produces very striking results in laser prints. Interesting effects can also be achieved with a laser printer by combining 100% black and 100% of cyan, magenta, or yellow.
Rich black is often regarded as a color that is "blacker than black". While this is nonsense from the point of view of color theory, the difference can often be seen in the printed piece. The difference can also be apparent in backlit (also known as "translite") pieces, where rich black more thoroughly blocks the light from coming through.
The use of rich black has to be based on a full understanding of the printing conditions, including the inks, printing press and especially the paper. If too much ink is used on poor quality paper such as newsprint, this may cause the paper to literally fall apart. In addition, excessive amounts of ink may not have a chance to fully dry before the printed result comes into contact with other pages. And the additional ink used to create rich black results in higher printing costs.
Care must be taken when using electronic design programs; for example, when managing a CMYK document in Adobe Photoshop, "black" may or not equal 100%K depending on the CMYK profile specified in the image's settings, and Photoshop will represent the various tones using RGB values close to black; whereas in an RGB document, "black" always equals RGB value (0,0,0).
Another reason to use rich black for small areas of black is to avoid trapping issues. Rich black is often used for text printed over a picture or colored background, because otherwise any slight mis-registration between printing plates would produce a white or colored halo around the text, making it much harder to read.
In some cases the process of preparing a work to print may include conversions to CMYK from RGB or other color spaces. The amount of black mixed with C,M,Y inks can be expressed as a process of under color removal or under color addition, though increasingly the entire conversion process is done using an ICC profile which expresses both the conversion and the under color handling.