I was sorting through some old resources that I made when I first began working and I found these colour paddles.
They were really simply made. I drew the template onto cardboard and painted them in the primary colours. Once the paint was dry, I glued matching cellophane onto the back and they were done.
I had to get them out for R to explore. He absolutely loved them and spent about half an hour looking at things with them. I also showed him how to put one on top of the other to create a new colour.