A bag of pompoms and some small plastic containers are useful when learning about sorting objects by color. I emptied the bag of pompoms in a larger container I had in the kitchen. Then opened six of the small plastic containers and put one pompom of each color into each of the containers. His job was to follow suit and finish sorting the colors into the proper container.
Once he sorted the pompoms by color, he dumped them out into the larger container and mixed them and resorted again. Then of course, he just wanted to play with them.
Supplies: I bought a bag of pompoms and a 10 pack of small plastic containers with lids from the dollar store, for a total of $2.