CMYK Colour Chart
The ink reduces the light that would otherwise be reflected. Such system is called subtractive because the inks “subtract” brightness from white.
CMYK is intended to give designers a quick tool for searching and comparing colours. Therefore make sure to always specify colours using CMYK-values in your layout program and you should not be selecting CMYK colours based on what you see on your screen unless you are choosing colours for electronic display only.