This article proposes a new algorithm for coloring a monochrome image. Recently, a few effective colorization algorithms have been proposed. Those algorithms can colorize monochrome images by giving them some color hints, and they work well as an intuitive impression. However, a realistic colorization in detail was difficult to achieve by those algorithms because they have not been constructed based on any physical model. They formulate a colorization process as a pixel-based digital signal processing. This article aims at developing a realistic colorization technique by considering a physical reflection model. We focus on the well-known dichromatic reflection model and try to colorize monochrome images by utilizing a property of diffuse-only reflection components. We demonstrate that high quality colorizations of still images may be obtained from a small number of color seeds given by a user.