The Unique Origin of Colors of Armchair Carbon Nanotubes
  The colors of suspended metallic colloidal particles are determined by their size-dependent plasma resonance, while those of semiconducting colloidal particles are determined by their size-dependent band gap. Here, we present a novel case for armchair carbon nanotubes, suspended in aqueous medium, for which the color depends on their size-dependent excitonic resonance, even though the individual particles are metallic. We observe distinct colors of a series of armchair-enriched nanotube suspensions, highlighting the unique coloration mechanism of these one-dimensional metals. 
E. H. Haroz, B. Y. Lu, P. Nikolaev, S. Arepalli, R. H. Hauge, J. Kono
cond-mat.mes-hall