What you need:
6 (clear) glasses
red food coloring
yellow food coloring
blue food coloring
How to amaze your little ones:
Fill the first three glasses with water. I used taller glasses for this step. Put red food coloring in the first, yellow in the second, and blue in the third. Watch the kids be delighted when they stir it in and the water changes the color.
Next, pour about 1/4 of each color into smaller glasses.
Add about 1/3 of what is left from the yellow water (oh, this sounds bad now...) from the tall glass into the smaller glass of red. Ooh and aahhh appropriately.
Repeat the step adding blue into the smaller glass of yellow. Ooh and aahhh again.
Repeat adding the red into the smaller glass of blue. Ooh and aahhh again (hopefully your purple turns out more obvious than ours did).
Final step: Laugh really hard (but quietly!) when your son takes a BIG breath and says, "It smells so good!" He is VERY susceptible to the power of suggestion. It looks like fruity drink, so it must BE fruity drink!