Complementary Colour
A complementary of a colour is the one that is found directly
opposite it on the colour wheel (red is the complementary of
green for instance).  Mixing a little of the complementary with a
colour will tend to “neutralize” or reduce the intensity of the
colour. A little red added to a brilliant green will produce a softer,
“warmer” green. Add some more red, and you get an “olive drab”
colour; some more, and the green starts to transition into brown.
Surprisingly, a colour, when juxtapositioned with its complementary,
appears to be intensified. This seems to work best when the areas
covered by the two colours are dissimilar – i.e., not 50/50.  The
two red squares at right are the same colour, yet the one set
against the green background appears more intense.

Use this idea in your paintings. If you need a colour to look more
intense, set it against its complementary. Use complementary
colours in the underpainting (ie. Some orange under a blue sky, or
patches of red underpainting peeking out through green grass in
the foreground), to increase the overall vibrancy of a picture.