There are lots of ways children can play and experiment with colour. Paint or coloured water are the obvious choices. In our home acrylic or poster paint with primary colours aways ends up brown before any other lesson is learnt. Water colour paints are wonderful to learn about mixing and blending colours. Montessori Schools of course would use a Colour Mixing tray similar to this. At home we have used coloured bottles, colour paddles. Here are a few resources we are currently using for mixing primary colours. Otis has almost mastered this so I am sure we will be exploring secondary colours soon. 

These are Color Flash Cards from Mr Printables. Not intended as overlays but I think they work fantastically well. For now I have only presented the primary colours red, blue and yellow. 

This is a primary colours, colour wheel which I have printed twice (on transparencies) and attached in the centre with a split pin/paper fastener. Otis can rotate the wheels to experiment and mix the colours. 

I love this above coloured dot sheet. Now that Otis knows how to use it he can do this work freely and with other colours once he is ready. This is free download from Teachers Pay Teachers Mixing Colors - Circles to Color. They are not my choice but Otis just loves using these dot markers. 

These Shapes and Colors Overlay Play Cards are also from Mr Printables. 

My children have always enjoyed using the colour mixing bottles for simple experimenting/playing on absorbent paper towel. I have coloured the water using food colouring. 

And to finish with some lovely play with light and colour blocks. Colour play is always lots of fun. Now I'm thinking how I extend this to secondary, tertiary colours and beyond - warm colours, cool colours, the full colour wheel.