First of all, put some white rice into some Ziploc bags - I used about two handfuls for each colour. Then drop in some food dye, seal the bag, and mix it all around. Try and spread the dye evenly, adding more drops of dye if necessary.
Children can help with this stage - as long as they can understand the instruction NOT to open the bags or CAN'T open them. F falls right between these two phases; he was utterly obsessed with opening the bags, and not interested in colouring the rice at all.
Once all the rice is coloured (and you can do as many different colours as you like), spread the rice out in a container. Unlike when I dyed the cooked spaghetti, the rice doesn't absorb the dye so fast, and benefits from drying out a little. But it is not too bad if you want to use it straight away.
The next day I gave the coloured rice to F in one dish, separated out into different colours. I also gave him some craft sticks to swirl the rice around.
He enjoyed slowly mixing up all the colours. I gave him some spoons as well so he could scoop the rice.
Of course he got all the colours mixed up, which was to be expected. But the rice still looked super pretty. And it'll keep indefinitely for future play!
**UPDATE** Three weeks later - still scooping and stirring!