A method is proposed to estimate spectral reflectances of watercolor paint samples with different water ratios. We first suppose that watercolor painting is monochrome made by a single paint and water. An algorithm is developed based on the Kubelka-Munk approach. The optical constants of scattering and absorption are estimated from the measured reflectances of a standard sample painted with a known water ratio. A linear relationship was devised to predict the optical constants of watercolor paintings with different water ratios. The surface-spectral reflectances are then recovered with the predicted optical constants. Moreover, we extend the method to watercolor paintings with mixed color paints. The feasibility of the method is examined in experiments using a variety of watercolor paintings.