Printed images are another story

In printing, the primary colours consist of the colours “in between” the RGB colours and are obtained by mixing. Apart from cyan, magenta and yellow, the contrast in printed materials is enhanced by black. Similarly, offset printing is based on the fact that the eye cannot differentiate between two adjacent dots of colour, but instead sees them as one.

The CMYK process colour model is said to be subtractive, as colours are created when inks absorb certain colours from light. As a result, the colours are reduced and only the required amount is left. In a subtractive system, cyan and magenta produce blue, cyan combined with yellow produces green, and yellow mixed with magenta creates red. The contrasting nature of the RGB and CMYK systems provides great opportunities for confusion when trying to recall the guidelines for colour mixing.
  Subtractive colour mixing. When all colours are combined, the result is black. 

Mixing paints is something else

A third situation arises when mixing paint or artists’ colours. Unlike in the previous examples, no attempt is made to deceive the eye into seeing two adjacent colour dots as one: the paint is instead made to be of a particular colour. As a result, the range of colours in paints can, in principle, be greater than in the case of printed or displayed images. Paint mixing is also a subtractive process, meaning that the paints partially absorb colours of light and leave the required colour.