Yes, you read that correctly. How to learn the colours with OOZE putty slime.
Here you can see how the finished resources turned out. The first thing I did was to laminate them, so that they are reusable and more durable.
These coloured foam sheets are ideal for teaching the colours to kids. They are cheap and versatile.
I found these at the 99cent shop, but you can find them in most art or craft shops.