Chameleons, particularly panther chameleons, can rapidly change color during social interactions. This color change is not due to pigment dispersion/aggregation but rather active tuning of a lattice of guanine nanocrystals in superficial iridophores. The study combines microscopy, photometric videography, and photonic band-gap modeling to show that chameleons shift color by adjusting the spacing of guanine nanocrystals in a triangular lattice. A deeper layer of iridophores with larger crystals reflects a significant portion of sunlight, especially in the near-infrared range. The two-layered iridophore structure is an evolutionary novelty that allows chameleons to combine efficient camouflage with dramatic displays and potentially provides passive thermal protection. The upper layer is responsible for rapid structural color changes, while the deeper layer contributes to thermal regulation by reflecting sunlight. The study also shows that osmotic pressure experiments can reproduce in vivo color changes, confirming the role of photonic crystal lattice adjustments. The findings highlight the unique structural and functional adaptations of chameleons, which enable their remarkable color-changing abilities.Chameleons, particularly panther chameleons, can rapidly change color during social interactions. This color change is not due to pigment dispersion/aggregation but rather active tuning of a lattice of guanine nanocrystals in superficial iridophores. The study combines microscopy, photometric videography, and photonic band-gap modeling to show that chameleons shift color by adjusting the spacing of guanine nanocrystals in a triangular lattice. A deeper layer of iridophores with larger crystals reflects a significant portion of sunlight, especially in the near-infrared range. The two-layered iridophore structure is an evolutionary novelty that allows chameleons to combine efficient camouflage with dramatic displays and potentially provides passive thermal protection. The upper layer is responsible for rapid structural color changes, while the deeper layer contributes to thermal regulation by reflecting sunlight. The study also shows that osmotic pressure experiments can reproduce in vivo color changes, confirming the role of photonic crystal lattice adjustments. The findings highlight the unique structural and functional adaptations of chameleons, which enable their remarkable color-changing abilities.