Get a big bag of multi-color skittles or other small colorful candy (ie. M&Ms, jelly beans - can also be done with small blocks, paper etc). Each student or group of students should have access to enough candies to complete the combinations. Explain how you can organize combinations by section: start with a specific order like red, red, and then change the last color and then concentrate on the next position - red, change, red. This is often easier if you arrange the skittles or colors on a graph paper backdrop.