I had a lot of food dye around from dyeing yarn when Lucy was born and when she got old enough I thought it was great that we could make our own coloured playdough. Home made playdough lasts so much longer than shopbought and I use this recipe and it feels great and lasts for a couple of months if sealed up between plays.
At some point Lucy was protesting about getting in the bath and so I added some colour to the water, just a half cap is enough to colour the whole bath and it did not dye her skin at all. We experiment in the bath by adding a colour at either end and seeing what happens when they mix, so we are learning through play. Also a coloured bath can make an enchanting setting for some water play with underwater animal toys etc.
Coloured Bath Soap for Kids
I have looked around for bath soap pens so Lucy could draw on the expanse of white tiles next to our bath without luck and at the suggestion of a friend I added some corn flour (starch) to some baby soap (you could use liquid handsoap for older kids), split the mix in half and added some food dye and voila! bath soap that you can paint on the walls or tub with the added benefit of getting a clean baby. Special note that I did dye my grout but it came out after a few scrubs
Pavement Paint for Kids
We made paint by mixing one cup of water with one cup of cornflour and spliting it into four bowls and mixing in some food colouring. BRILLIANT! This is my favourite thing to do with food colouring and Lucy loved it too. We got the idea from the Tend website. The colours blend together brilliantly when wet and then change colour to a more opaque/chalky texture as they dry and you can just sweep/hose off the results. We didn't have any residual dyeing on our cement pavers.
Ice Cube Painting
On a hot day you can freeze some water that has been dyed with food colouring and then pop it out and let your kids use it like a paintbrush. This is another great chance to see colours combine and observe the result. Something simple and cheap that can keep kids entertained and cool on a hot day.
We did make home made fingerpaints which I posted about here, but they were no where near as fun as the pavement paints, which you could use as fingerpaints if you wanted to, but not for paintings you wanted to keep.