Heat the water to the desired temperature (above 160°F or 71°C).
Then stir into the flour, aiming to remove any lumps. A whisk works great here but a fork can also do the trick.
Last, stir in the food coloring and you’re ready to paint!
Notes
Gluten-free flour can also be used instead of all-purpose. Heating raw all-purpose flour to 160°F (71°C) kills bacteria that may be present. I often boil water to way beyond this point, like 200°F for example, just to be extra safe.