Have the children sit around the a table and give each a water bottle (with the cap!) and ask one child what two primary colors they want to add to the bottle. Have the child help squeeze the food dye (2 drops each)into the bottle. Then have them screw the cap on and mix the bottle around so they see the colors mix. (follow the same steps for the other children.)
Have the other children watch and observe what happens when the two colors are mixed. If you want to you can make a chart of the primary and secondary colors.
**When i was student teaching my pre-k students loved this and we went through atleast 10 bottles.