Real time, in situ Raman spectroscopy with two simultaneously incident laser excitation wavelengths is used to investigate the dynamics of single-walled carbon nanotube etching. For the source material, nanotubes of diameter 1.4 nm, a 532 nm laser is resonant with semiconducting nanotubes and a 633 nm laser is resonant with metallic nanotubes. Changes in metallic and semiconducting population are tracked separately and simultaneously. In oxygen, metals consistently etch faster than semiconductors, and all etch rates increase with the process temperature and the defect density in the source material. A similar evolution is observed in carbon dioxide. Simultaneous two-color Raman spectroscopy provides information beyond standard Raman spectroscopy and can be effective as an instantaneous measure of metallicity for nanotubes.