Cut a disk a few centimeters or so in diameter from a piece of cardboard; punch two holes a bit off-center, big enough to loop a piece of string as shown in the sketch.
Twirl the disk as shown, so the string winds up like a rubber band on a model airplane. Then, if you tighten the string by pulling outward, the disk will spin. If half the disk is colored yellow and the other half blue, when it is spun the colors will be mixed and appear nearly white. (How close to white depends on the hues of the colors.) Try this for other complementary colors.