This paper describes a method for coloring voxel-based models. The method generalizes the two-part texture mapping technique to color non-convex objects in a more natural way. The method was developed for coloring internal cavities for the application of virtual endoscopy, where the surfaces are shaped like a general cylinder in the macro-level, but with folds and bumps in the more detailed levels. Given a flat texture, the coloring method defines a mapping between the 3D surface and the texture which reflects the tensions of the points on the surface. The core of the method is a technique for mapping such non-convex surfaces to convex ones. The new technique is based on a discrete dilation process that is fast and robust, and bypasses many of the numerical problems common to previous methods.