Pour 2 tablespoons of water into a small container or one of the cavities of a muffin tin. Repeat for as many colours as you're planning on making.
Add all-purpose flour and stir until combined.
Add clear dish soap and stir until the mixture is smooth and free of clumps.
Add liquid watercolour paint, stirring until the colour has been evenly distributed throughout. Use immediately.
Notes
Window paint can be wiped off with a paper towel while still wet. If the paint has had time to dry on the window, use a wet cloth to soften and wash it off.