The colors of armchair carbon nanotubes—so named for the arrangement of their honeycomb-like carbon building blocks—are determined by factors different from those that color other nanomaterials, researchers report (J. Am. Chem. Soc., DOI: 10.1021/ja209333m). Size-dependent colors in nanomaterials are common, but underlying electronic properties usually provide the color explanations. For instance, the color of metallic gold nanoparticles depends on the wavelength at which their free electrons collectively oscillate, known as plasma resonance frequency. And semiconducting nanoparticles, or quantum dots, have the colors they do because their size changes the energy gap between their key molecular orbitals. Erik H. Hároz and Junichiro Kono of Rice University and their colleagues sorted sets of armchair nanotubes and obtained their absorption and Raman spectra. Although the nanotubes are metallic, their colors don’t depend on plasma resonance, the researchers learned. Instead, the color depends on how light is absorbed by high-energy electron-hole pairs called excitons, and it differs depending on the diameter of the tubes.