Here is an inexpensive, easy and fun craft project that you can prepare ahead of time and have ready for your kids and their friends to use, or you can let supervise the kids as they make it on their own. This is a recipe for making your own sidewalk chalk "paint". The so called paint really does look like chalk when it dries, and if the sun is out and it is warm, then it dries pretty quickly.
You Will Need (per each color):
1/4 Cup Cornstarch
1/4 Cup Water
Food Coloring colors of choice
Mix the cornstarch and the water together, when thoroughly mixed (make sure all lumps are worked out), add the food coloring and mix it thoroughly. Be sure to add the food coloring one drop at a time and use the minimum number of drops to achieve the color you want. Remember that food coloring, when used in excess, can stain skin, and children are notorious for painting themselves as much as anything else! As long as you use it in moderation, there is no problem with washing it off. If you have Repeat real small ones, or don't trust yourself with the food coloring, try using some of your soaping herbs for a natural colorant. They don't give quite the colors you get with the food coloring, but it's your choice.
Anyway, repeat this process for each color that you want to make, then go on outside and paint! When you are done, cover the paint and store in the fridge for another afternoon of sidewalk painting! I am not sure how long the shelf life is on this paint, it's never sat around long enough to test! But since it does have water in it, we know that it will grow nasties at one point or another. I recommend keeping it refrigerated and only making what you will use in a week or two. You may however, find that it stores longer, but I error on the side of caution, especially where children are involved. Be sure to shake or stir the paint well prior to use, and make sure to keep it stirred while you are using it, because the cornstarch will settle to the bottom of your container.
If you want to make it easier to use several colors, try putting them in a multi-compartment something. Try a soap mold, a muffin tin, a brownie pan, or even an ice cube tray would work, although you may need two or three sections for each color. Use your imagination as you look through your kitchen cabinets!