CMYK means Cyan, Magenta, Yellow and Black, where Cyan, Magenta, Yellow are the primary colors in subtractive synthesis. From a theoretical point of view, every possible shade from black to white could be reproduced by combining the CMY colors. In the reality, the lack of purity of the inks used for printing process (e.g ink jet or offset) make it impossible to reach this theoretical black.

When adding the maximum quantity of CMY, only a dark brown color can be printed on paper. Therefore, a 'K' or black ink was added in order to get the deepest and purest possible black. A CMYK file consists in 4 composite layers (each will be coded from 0 to 100).

Caution: Even if the ink jet printers use subtractive CMJK synthesis, we recommend you to work with an RBG file until you finally print the image.
This will allow keeping the maximum achievable gamut. All the pigment-based ink jet printers can work with 2 types of black inks as standard : one 'black mate ink' is usual devoted to mate papers, while the other 'black photo ink' is solely used for glossy or semi-glossy papers.
Depending your printer and the type of paper, the choice of the black ink can be either automatically or manually decided through the driver configuration. Please pay particular attention to our recommendations on printer settings linked to the proposed ICC profiles.
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