1. Wrap shoe boxes, toilet paper rolls, and anything else that would be "buildable" in colored construction paper.
2. Put all of the building supplies in the middle of the room.
3. Give your child a suggestion. Like, "Find all of red blocks and build something in front of something else red." Your child can use his imagination to build something big or small.
4. Give another color suggestion.

You'll be amazed at the kinds of things he'll find around the house, and he'll learn colors quickly.