Rich black
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Rich black is a term used in printing to refer to a mixture containing all four of the cyan, magenta, yellow and black (CMYK) colors. Less frequently it could be a mixture of black and some other ink. A typical rich black mixture might be 100% black ink and 50% of each of the other 3 inks. This is necessary because no black ink is truly black and the mixture helps to make the black even more 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 it may simply fall apart. In addition, excessive amounts of ink may not have a chance to dry before the printed result comes into contact with other pages. Finally, rich black, because it uses more ink, will have higher costs.
Care must be taken when using electronic design programs. Photoshop's "default" black (the color used when the 'D' key on the keyboard is pressed) is not typically 100K. (Though in RGB documents, 00-00-00 is used.) The value chosen for black in CMYK mode depends on the CMYK profile chosen in color settings. If you make a document in Photoshop with various blacks, Photoshop will actually use different not-quite-black RGB values to draw them on the screen. Adobe Illustrator and InDesign, on the other hand, default to 100K.
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.
