Learning to mix colours is a fascinating experience for children as it combines art and science. They are always filled with wonder every time they see the colours mixing.
I don't specifically teach colour mixing in separate lessons but incorporate it into their art/craft/English lessons (whenever painting is required).
There are only 5 colours of paint that I buy: blue, red, yellow, white and black. At first, I provide children with 2 primary colours (eg blue and red) on their tray and let them paint using the 2 colours. They are allowed to mix them. I ask questions: what 2 colours did we start with? What colour do we have now (purple)? After a few experiences with this I provide them with 2 other primary colours (eg blue and yellow to make green). The same things happen then I provide them with the 2 remaining colours (eg red and yellow to make orange). So over the weeks they would have plenty of experiences mixing primary colours to make secondary colours.
After this, I introduce white to their palette. I show them that white makes everything lighter. From this point on, every time we need to paint, I only provide them with 4 colour paints: red, blue, yellow and white. They are free to use and mix as they please. They have become experienced with mixing and produced some wonderful colours.
The following are a few examples of what started out with the usual 4 colours: red, blue, yellow and white, but each palette ended up with different colours.
Look at all the lovely colours!
I love the different shades of blue in this picture!
It was painted using one of the trays above.
You might wonder about the black paint that I mentioned earlier: later I would introduce black and ask them what they think black would do to the colours. I showed them that if white makes everything lighter then black would make them darker. The children didn't really like adding black! It made everything dark and dull. They preferred the white so I stopped providing them with black altogether.
If you follow my colour mixing 'lessons' then you would never need to buy paints other than in the colours of red, blue, yellow and white (and maybe black)!