Draw a red or blue dotted on your left hand. Go to school and tell others to do the same. If they want to draw a blue dot, tell them to draw it on their right hand.

Left hand: red dot
Right hand: blue dot

It must be these colors

Tell them to tell their friends to do the same thing. A while later, walk around and see has which colored dot on their hands. Than(the best part) start a color war by saying one is better than the other.

You can also do this with numbers.

Left hand: 94
Right hand: 49

It must be these numbers