I introduced the concept of contrasting colors by using an orange and a blue and orange piece of foam paper. We learned that background is important! You can see an object much better if the background color is much different (or contrasts with) the object.
Then I gave the kids a box of different items and some different colors of foam paper. They tried each object on the papers and decided which objects showed up best on the different colors. We learned that generally speaking, cool colors show up better on warm colored backgrounds and warm colors show up better on cool backgrounds. We will solidify this idea more later on when we talk more about warm and cool colors (looking back, I should have covered that first, but forgot!).
Later, I gave the kids several different colors of construction paper and the same colors in smaller rectangles. They experimented with contrasting the small rectangles with the bigger ones and once they were satisfied, they used a glue stick to glue their choices down. Finally, I stapled their choices into a little book.