Click on the image below to view a much larger version.
The color wheel above shows three colors in addition to the primary colors. In between each pair of primaries is a secondary color: a mix of two primaries. Cyan and magenta make blue. Magenta and yellow make red. Yellow and cyan make green.
I’ve shown a formula for each color. The numbers are the respective percentages of Cyan, Magenta, Yellow and black (K) in each hue. These are the numbers you would put in as the CMYK values in the Color Picker.
Mixing CMYK Color
When you add cyan, magenta, and yellow together, you get black – in theory. In reality, it’s a murky soft black, so printing a rich black requires the addition of black ink, referred to as the Key color. Mixing these four colors together in various ways gives us almost all the colors in the rainbow.
Out of Gamut
Many colors which are visible to the naked eye are not reproducible on a printing press with CMYK. These colors are called “out of gamut”. We can get pretty close, though. Photoshop and Illustrator have commands you can use that bring your colors back into gamut, so that they print a predictable color.
In the Color Picker, look for an icon of a little triangle, with an exclamation point inside, next to the current color: If you see that icon, your chosen color will not print in CMYK. Click on the triangle to choose the closest in-gamut color.
Four-Color Process Printing
In four-color offset lithography (also known as process printing), a full-color image is separated into four plates. These plates look somewhat like photo negatives after they are etched. The black areas of the plate hold no ink. Conversely, the lighter the etched area on a plate, the more ink it will hold. There is one plate each for cyan, magenta, yellow, and black.
Plates printed for process printing such as these are not printed with solid ink*. They are actually comprised of patterns of dots we cannot distinguish with the naked eye. Try looking at a magazine ad through a magnifying lens some time. It’ll look something like this:
Each of the four plates is printed in a grid of dots. So that the dots are not overlaid exactly on top of each other, the grid for each plate is turned at a different angle. The black grid is at 45°, magenta is at 75°, yellow is at 90°, and cyan is at 105°. When printed together, they form a rosette pattern.
*Solid ink is printed when we use spot colors.