If the color of hydrate were identical to the color of the anhydride, how could you be sure that you had driven off all the water from the hydrate?

Answer is: you can calculate the number of moles of water in a hydrate if you know the empirical formula of the anhydride.
First measure mass of hydrate, after you heat hydrate, put sample of the compound on scale and measure the mass loss. The lost mass is water. Now  calculate the number of moles of water and if the ratio is same as empirical formula, all water is lost.