First we need to choose some beautiful bottle or jar with a screw cap.
Pour on a clean sheet of paper a small amount of fine salt (about 1/6 or 1/7 of the total volume of the vessel).
Take particular color chalk and rub salt with it. Now salt gets colored granules
Carefully pour the resulting color salt in a glass jar.
Take a blank sheet of paper and repeat the same procedure with a different color chalk.
Entirely refill the bottle with salt layers of different color. Pour salt into the container at an angle by rotating the vessel. Just do it very carefully so as not to mix the layers, otherwise, instead of the rainbow in a bottle you get a mixture of dirty brown.