Subtractive color mixing:  light colors are subtracted from one white light beam
  blue  + yellow  =  green
white light beam =
sum of all spectral colors
This blue
reflects
no violet.
light
reflected
by both
blue paint
(cerulean)
shining on:
reflects
a brighter
green, less
dull,
because the
(violet +
orange =
brown) is
absent
yellow paint
(lemon yellow)
This
yellow
reflects no
orange.
One light beam!!!!