This was a huge hit at our house and we will definitely be doing it again!
I've seen lots of variations but I used 1/2 cup of cornstarch to 1/2 cup of water. Then mixed in food coloring.
It mixed easily but separated after a while and needed to be remixed occasionally.
The boys dug right in and had an absolute blast with this stuff.
They started on the sidewalk but quickly begin painting everything else in the yard. Trees, slides, toys....
.....their boat. I didn't get any after pictures but the top of their boats was covered from one end to the other in a rainbow of colors.
Once it dries it's a light powder that wipes right off, or washed away after the first rain. Cheap, easy, and safe.