The Four Colour (Full Colour) Printing Process is universally used in the graphic arts and commercial printing industry for the reproduction of colour images and text.
The Four Colour Printing Process or CMYK refers to the four inks used in colour printing: Cyan, Magenta, Yellow, and Black (the K in CMYK).
The Four colour process printing, or CMYK printing, is a system where a colour image is separated into 4 different colour values (called a colour separation) using graphic arts software. The result is a colour separation of 4 images that when transferred to 4 different printing plates and sequentially printed on a printing press with the coloured inks Cyan, Magenta, Yellow and Black (the K in CMYK), reproduces the original coloured composite image.
In the photograph of the landscape you can see the different values of Cyan, Magenta, Yellow and Black when colour separated.
Most of the entire spectrum or of colours are reproduced with just the four process ink colors.