This paper presents a new technique for automatically converting digital images into colored pencil drawings. In real colored pencil drawing, artists use different colors for different regions, and add pure colors directly onto paper to build the target color through optical blending. To support such effect, our technique extends the existing technique for reproducing color image with custom inks to automatically select the best color set for individual regions in a source image. Then layers of stroke image for each color are generated and superimposed with the Kubelka-Munk optical compositing model. We also allow users to specify regions and to customize the color set for a specified region interactively. The proposed technique can be easily extended to simulate other artistic media featured with optical color blending, such as pastel and wax crayons.