We cannot give you an exact answer as this will always depend on how long or thick your hair is.
If your hair is of average thickness and about shoulder length, you will usually need 1 vial of Water Colour Gel. For longer or thicker hair you may need 2 or even 3 vials.
If you are applying to your roots only, you may even get 2 applications from the same vial of hair colour gel. You will need to split the powder carefully and reduce the water volume. Ensure any powder you are not using right away remains totally dry and is tightly re-sealed in the vial.
Our friendly support team are here to help, so please do contact us.