"Tertiary" colours can also be a mixture of all 3 primary colours

Russet = mostly red + little blue and yellow

Olive = mostly blue with a little yellow and red

Citrine = mostly yellow with a little red and blue.

Note that the intensity of the colour becomes greyer as the primary colours are mixed.  

 



This can be seen as you mix the primary colours together

Russet = mostly red + little blue and yellow

Olive = mostly blue with a little yellow and red

Citrine = mostly yellow with a little red and blue.

Note that the intensity of the colour becomes grey as the primary colours are mixed.

If equal amounts of primary colours (red, blue, yellow) are mixed a perfect grey is created.