Here is what you do: Put 6 containers/bowls together. Put about a large spoonful of yoghurt (if you want to learn about how to make yoghurt at home, click here) in every second bowl leaving the ones in between empty. Add some food colouring into each bowl with yoghurt and then stir it until the yoghurt takes the colour of the food colouring. After that, in the empty bowls, make mixtures. So, it goes like this: In the empty bowl between the red and the green, you (or your child) should mix a bit of red and a bit of blue yoghurt and so forth. Then, let your child stir the two colours together and see what colours they make.
Depending on your child's age, your child may only need your supervision from the beginning to the end for this activity. The rest, she may be able to do all by herself! You just sit back and enjoy the amused face of your precious one.