6-8 Drops Food Coloring
We used neon, which was super-bright and worked really well. I threw all the stuff they'd need in a bag/containers, gave them some paintbrushes and out the door they went. They especially loved mixing the paint themselves- this couldn't be easier.
At first they painted pictures, then Grace started splattering, which looked so cool because the paint is dark when it's wet then after a few seconds on the hot concrete it dries bright and electric. That ended up being so fun, they Pollock'd until the paint [all my constarch] was gone.
Lily turns to me and says, "You can eat food coloring. And bake with cornstarch. And we drink water. So...."
See where she was going with that?