First of all it’s useful to know that most artists (digital artists being an exception) work with ‘reflected’ light as opposed to ‘emitted’ light.  Emitted light comes directly from a light source such as the sun or a pc screen whereas reflected light is emitted light that has been bounced (or reflected) off a solid surface – in the artist's case, canvas, board or paper for example.  All light travels in waves and it is differences in the wavelength of the light that alter the colour we perceive. Any given wavelength, whether its source is emitted or reflected, always produces the same perceived colour but that is where the similarity ends.

The weird fact, which everyone seems to just accept but I for one do not understand at all, is that the primary colours for emitted and reflected light differ.  The three primary colours (i.e. those that cannot be created from any others) for emitted light are red, blue and green, whereas an artist’s primary colours (the reflected ones) are red, blue and yellow.  Also, if you mix the three primary emitted light colours together you get white light, whereas if you mix the three primary reflected colours together you get a very dark brown paint mix (actually theoretically black).  All very bizarre but not really of relevance here so I’ll move rapidly on to discussing artist’s colours.