Try this - it amazes the kids!
Get a nice bunch of white flowers with plenty of open buds - carnations work best.
Half-fill some jugs/glasses/anything that'll hold water and support stems and add a good squirt of food colouring to each one, making sure each is a different colour.
Add your flowers, wait a couple of hours (that's all it takes), then check out your 'new' blossoms!
For even more 'WOW' factor, split a stem to approx 6 inches up and place each half in a different colour jar!!!
Enjoy!
Also posted in 'Colours, Shapes, Numbers and Letters' and 'Sciences and Processes' with a scientific explanation!