Mix equal amounts of cornstarch and water together (I think I did half a cup of each). Add some dish soap (just a squirt or two). Mix. Pour into ice cube trays and add food coloring. Mix the color into the mixture. Freeze.
I used the gel food coloring. I'm not sure if the drops would make vibrant colors. My hands did get slightly colored but not too bad. The lady on Reading Confetti said that their hands got really stained and so she was going to try washable paint next. The washable paint does not come off the driveway easily but the food coloring does. So it's your choice!
When you color on the driveway, the chalk will appear wet and not colorful but as it dries, the color shows up. Take a look.
Then we put the ice cubes in a row to see how they melted. Clearly my driveway must slope to the left. These ice cubes were in a straight line!