Recent research has shown how to change the color of existing objects using photochromic materials. These materials can switch their appearance from transparent to colored when exposed to light of a certain wavelength. The color remains even when the object is removed from the light source. The process is fully reversible allowing users to recolor the object as many times as they want. So far, these systems have been limited to single color changes, i.e. changes from transparent to colored. In this paper, we present ColorMod, a method to extend this approach to multi-color changes (e.g., red-to-yellow). We accomplish this using a multi-color pattern with one color per voxel across the surface of the object. When recoloring the object, our system locally activates only those voxels that have the desired color and turns all other voxels off. We describe ColorMod's hardware/software system and its user interface that comes with a conversion tool for 3D printing as well as a painting tool that matches physical voxels with the desired appearance. We also contribute our own material formula for a 3D-printable photochromic ink.