Physiological color change and geographic variation in coloration are well documented in several squamate lineages, having presumably evolved for cryptic, sexual signaling, and thermoregulatory functions. Only 11 snake species have demonstrated physiological color change, although anecdotal reports suggest it may be present in additional species. We describe color variation and physiological color change in island and mainland populations of Boa constrictor using full-spectrum reflectance spectrometry. We employed principal components analysis (PCA) summarizing the spectrometry data into two axes that describe the brightness (PC1) and chroma or relative intensity of particular wavelengths (PC2). Boas from island and mainland localities exhibited physiological color change, and this change occurred on a diel cycle. Boas from both locations were lighter at night and darker during the day. The magnitude of the color change differed between our two PC axes. Although change in brightness was similar for boas on the islands and the mainland, the change in chroma was greater in boas from the mainland. Color also varied seasonally; boas were lighter in color and reflected more long-wavelength light during the wet season than during the dry season in Belize. We suggest that a fundamental hormone cycle (melatonin/melanophore stimulating hormone, MSH) present in a wide variety of vertebrates, underlies the physiological color change in snakes. If this is true, color change may be more widespread than previously realized, and the perceptual bias of the human vision system may have caused researchers to discount its presence in snakes.