This is one of the best activities yet! Yesterday we took 4kgs of rice and divided it into 4 large ziplock bags. We put a generous splash of water into each bag, sealed it and squashed it around so all the rice grains were wet then we added a generous splash of food colouring and mixed it around again until it was well coated.
We then put it on trays to dry over night. I lined the trays with foil in case it got stained but it wasn't necessary cause the rice held the colour very well.
I put the rice into a large plastic container that has a lid so now we can store our rice container and play with it whenever we want.
Of course Andy played with it first thing this morning and the colours stayed separate for about a split second but it looks even prettier when mixed.
I bought a mini rake and spade and funnel for him to play with but be collected all his toy animals and insects and they're all having a great time in the rainbow rice (he's playing with it now as I type). You could also make it into a discovery center and hide things in it for them to dig out and find.