Two
colors that when
added together produce
white.
Complementary colors are on
opposite sides of the
color circle, for example
yellow and
blue or
violet and
green.
However, if you try making white paint by mixing yellow and blue you're bound to be disappointed, because mixing paint is not the same as adding colors. Instead, try pointing yellow and blue lights at a white wall. You'll see that where the light cones overlap, the mixed light is white.
If colored lights are hard to come by, you can try the same with a paint program like the GIMP or Adobe Photoshop, or even Paint Shop Pro, by changing the mode from Normal to Additive and paint on a black background. You now paint with light.