Many factors can contribute to geographic color variation, including natural selection, sexual selection, and genetic drift. In squamates, phenotypic plasticity can also play an important role in color variation because color can change during maturation, across seasons, and over short time scales in response to environmental stimuli. Here, we quantify geographic color variation and assess the contribution of physiological plasticity (rapid color change) in populations of Eastern Collared Lizards (Crotaphytus collaris). First, we quantified the dorsal coloration of four geographically distinct populations of C. collaris: two putatively melanistic populations on lava flows, and two typically colored populations from the surrounding desert. Second, we quantified the degree of physiological color change occurring in response to stress and temperature, two factors known to affect reptile coloration over short time scales. We found that populations differed in overall coloration, but that only one of the two lava-flow populations was darker than its neighboring population. Both lava-flow populations exhibited less contrast with their respective substrates than the desert populations. All populations exhibited some physiological color change, but the degree of color plasticity was narrower than the differences in color across populations. Future studies on the genetics of coloration will be necessary to understand the specific mechanisms contributing to color variation in C. collaris, but our results indicate that natural selection for background matching likely plays a role in this system.