Very simply, take equal amounts of baking soda and cornstarch and add it to the bottle. (I use a rough three tablespoons of each). Than, add food coloring. Obviously the more you use, the more intense the color. Finally, fill the rest of the bottle with water and . . shake! Shake hard and for a minute or so - make sure that the powdered ingredients are mixed well. Make as many colors as you want but also fill one bottle for each child with Vinegar. Once they have squirted their paint, they can then squirt it with vinegar and watch the reaction.
I found these bottles on Amazon. I checked out the Dollar Store and Walmart and couldn't find any clear squirt bottles. . .. probably next week, I'll find them for $1.00 everywhere HA!
I was a little worried that the color wouldn't wash off the sidewalk but, we got a good rain on Monday night that washed it all away so, for the record, it will all wash off with water!
Next time, I would add the baking soda and cornstarch to the bottles before we leave for camping - it was super messy to fill the bottles outside not to mention having to carry huge boxes of each item.