The CMYK colour model (process colour, four colour) is a subtractive colour model, used in flexo printing,
and is also used to describe the printing process itself. CMYK refers to the four inks used in some colour
printing: cyan, magenta, yellow, and key (black). Ink is typically applied in the order of the abbreviation.

The "K" in CMYK stands for key because in four-colour printing, cyan, magenta, and yellow printing plates
are carefully keyed, or aligned, with the key of the black key plate.

The CMYK model works by partially or entirely masking colours 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.

With CMYK printing, halftoning (also called screening) allows for less than full saturation of the primary
colours; tiny dots of each primary colour are printed in a pattern small enough that human beings
perceive a solid colour. Magenta printed with a 20% halftone, for example, produces a pink colour,
because the eye perceives the tiny magenta dots on the large white paper as lighter and less saturated
than the colour of pure magenta ink.

Without halftoning, the three primary process colours could be printed only as solid blocks of colour, and
therefore could produce only seven colours: the three primaries themselves, plus three secondary colours
produced by layering two of the primaries: cyan and yellow produce green, cyan and magenta produce
a purplish blue, yellow and magenta produce red (these subtractive secondary colours correspond roughly
to the additive primary colours) plus layering all three of them resulting in black. With halftoning, a full
continuous range of colours can be produced.

Typically you would request a 4 colour process job if you are trying to recreate a photo like art or image,
as well as any images with screens and gradients.