Seal one end of cardboard tube with foil.
Mix water and food color in a medium bowl or disposable plastic container. Add to plaster of Paris to make a "pudding" consistency. Stir until the mixture starts to thicken. Work fast!
Stand your cardboard tubes on end (on wax paper or foil.) Pour the mixture into the tube.
When firm, peel off the cardboard. Any cardboard left on chalk can be rubbed off with gloved finger or scraped off with knife blade.
Dry overnight. Go out to your sidewalk or driveway, and start drawing!