The CMYK color model (process color, four color) is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black). Though it varies by print house, press operator, press manufacturer, and press run, ink is typically applied in the order of the abbreviation.
The "K" in CMYK stands for key since in four-color printing cyan, magenta, and yellow printing plates are carefully keyedor aligned with the key of the black key plate. Some sources suggest that the "K" in CMYK comes from the last letter in "black" and was chosen because B already means blue. However, this explanation, although useful as a mnemonic, is incorrect.
The CMYK model works by partially or entirely masking colors on a lighter, usually white, background. The ink reduces the light that would otherwise be reflected. Such a model is called subtractive because inks "subtract" brightness from white.
In additive colour models such as RGB, white is the "additive" combination of all primary colored lights, while black is the absence of light. In the CMYK model, it is the opposite: white is the natural color of the paper or other background, while black results from a full combination of colored inks. To save money on ink, and to produce deeper black tones,unsaturated and dark colors are produced by using black ink instead of the combination of cyan, magenta and yellow. - WIKI
CMYK - THE COLOUR FORMAT FOR PRINTED, FULL COLOUR IMAGES:
CMYK stands for “Cyan, Magenta, Yellow and Black”, and CMYK colour – sometimes referred to as “4-color process” or “process” color – is used specifically to produce full-color graphic materials and photographs on a “4-color” printing press. The “4” refers to the number of inks used (cyan, magenta, yellow and black) and “process” refers to a special printing technique that recreates the full spectrum of visible colors using just these four ink colors.
A graphic or photograph that was printed using CMYK color is made up of thousands of overlapping little dots of ink that the press puts down on paper as the paper passes through it. And – yup, you guessed it – those dots are cyan, magenta, yellow and black in color. The angle and density of these different colored dots relative to each other, and the diameter of the dots themselves, affect the color being displayed in a particular part of a printed document.For example, large yellow and cyan dots placed together without any magenta or black dots nearby will make that part of a document look green. Large magenta and cyan dots with no other colored dots nearby will produce a purple or dark blue. Large dots of all four colors placed together will create a very dark, rich black.
The size of the dots, both in actual dimension and in relation to one another, will change the saturation and hue of the colour. By adjusting the size of each dot relative to the other and the size of the dot to allow more or less of the background show through, you can recreate any color from a light lime yellow to a dark green-blue, and everything in-between. If you add a black dot nearby, adjusting its size and its density relative to the yellow and blue dots will allow you to make the color darker or lighter. If you add magenta, you’ll start to move the color towards any other number of colors depending again on the number of magenta dots relative to the yellow, blue and black dots, and the size of the magenta dot relative to the yellow, blue and black dots.
The graphic below is a good visual explanation of how 4-color process printing works to achieve different colour results.
THREE EXAMPLES OF COLOR HALFTONING WITH CMYK SEPARATIONS. FROM LEFT TO RIGHT: THE CYAN SEPARATION, THE MAGENTA SEPARATION, THE YELLOW SEPARATION, THE BLACK SEPARATION, THE COMBINED HALFTONE PATTERN AND FINALLY HOW THE HUMAN EYE WOULD OBSERVE THE COMBINED HALFTONE PATTERN FROM A SUFFICIENT DISTANCE.
In essence, 4-color process printing uses the same color principles you learned as a kid in art class. Just like mixing the primary colors (red, blue and yellow) on an artist’s palette allowed you to create an entire spectrum of other colors to fingerpaint with, “mixing” our 4 “primary” printing ink colors (or adjusting the size, angle and relative density of each of the 4 colored dots throughout a document) allows a printing press to recreate a good portion of the visible color spectrum, and produce a “full-color” image.
When graphic designers create graphics meant to be printed in full-color – like for brochures, catalogues, magazines, and even for full-color logos or business cards – they “set” the colors for the graphics they create as “CMYK”. This way, when a printing press outputs these files as printed materials, it understands the right density and sizes for each of the four colored dots throughout the document in order to achieve the final desired result.