mixing light and material colors

two blue spheres, a white wall, a blue light, and a yellow light

mixing a light with its complementary material color results in a complete cancellation:

  1 1 0 yellow
* 0 0 1 blue
-------------
  0 0 0 black

multiple lights mix first with the surface color, then are summed.
  1 1 0 yellow       0 0 1 blue
x 1 1 1 white      x 1 1 1 white
--------------     --------------
  1 1 0 yellow       0 0 1 blue

            1 1 0 yellow
          + 0 0 1 blue
          -------------
            1 1 1 white

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