This is a favourite for the children I work with, even the older ones!
You will need: 3 colours of water (ideally yellow, red and blue). The easiest way to do this is to use food colouring and water and mix them together, but paint and water works just as well. 6 cups (3 with the water colours in and 3 empty). A jug of water. A pipette or old medicine syringe is helpful but not essential.
Pour some plain water into a cup (preferably see through). Use the syringe/pippette to add one of the colours into the water. Now add another colour. Mix them with a stick. What colour do they make?
It is a good idea to start simply and mix:
Yellow and red
Red and Blue
Blue and Yellow
Once the children have the idea, they can play about and see how many colours they can make! It's useful to talk about primary colours and why they are the basis for all other colours... if you're not sure, there is a video here.