Have your kids look up "opponent-process theory of color vision." Here's an
attempt at explaining it. The reason you see the opposite colors is because
receptor cells in the retina are linked in pairs that work in opposition to
each other--red and green are paired, blue and yellow and black and white.
When the red receptor cells is firing off info because you're looking at red,
it inhibits its paired green cell from firing. When you look at white, both
paired cells should fire, but since you were just staring hard at red, the red
cell is tired and less able to respond. The green one is not tired at all and
it fires off as normal so you see the green component of that white light. The
green fades as the red cell recovers from the fatigue and starts firing again.
