The Red/Green/Blue colour mixer
You may hove noticed that certain colours on your PC screen seem particularly vivid - the Red, Green and Blue in the colour circles in figure 1 below are particularly bright. This is because the Red, Green and Blue phosphors used in creating the image on your screen produce light which is very close to the particular frequencies to which the human eye's sensors are most sensitive.
Figure 1 - the Red/Green/Blue colour circles
Figure 1 above shows an approximation the red, green and blue primary colours (the additive primary colours), and the effects of mixing them. Some of these are not intuitive - when Red is mixed with Green, the result is Yellow, not Brown. When Blue is mixed with Yellow, the result is White, not Green - yet we all remember mixing Blue and Yellow paints to make green as children.
The Red/Yellow/Blue colour mixer
Figure 2 - the Red/Yellow/Blue colour circles
The colour circles in Figure 2 describe roughly the colour mixing we all remember. The explanation for the difference is that Figure 1 describes the effect of mixing pure light of single frequencies (colours) at the eye. Figure 2 describes the effect of light being reflected from mixtures of coloured pigments. The pigments used in paints and crayons are not single colours of light. They are mixtures of substances, reflecting mixtures of colours, which are interpreted by the eye to the nearest colour that it can interpret.
The Cyan/Magenta/Yellow colour mixer
Another set of colour circles in common use is the Cyan/Magenta/Yellow mix commonly used by printers (the subtractive or absorbtive primary colours):
Figure 3 - the Cyan/Magenta/Yellow colour circles
This set of colour circles indicates the way that cyan, magenta and yellow inks absorb light on a white paper background.