This chapter explains atomic and molecular origins of color in insulating materials. For many, the colors in nature and in the laboratory are one of the inspirations for studying science. Color in materials is caused by interactions of light waves with atoms, and especially with their electrons. In fact, color is a manifestation of many subtle effects that are important in determining the structure of matter. Color is often the result of transitions between electronic states; this can involve energy absorption or emission. A well-known example of atomic emission is the yellow color of sodium in the flame test for the element. Other examples of colors arising from atomic emission include the colors of lasers making use of monatomic gases. The loss of the proton can be detected visually, as the color of the acid and its conjugate base can be substantially different, especially due to different degrees of delocalization.