The color of niobium phosphate glasses ranges from clear to deep blue depending on their thermal history. In this study, a glass with the nominal composition (mole %) 23K2O·40Nb2O5·37P2O5 was treated in oxidizing and reducing atmospheres at temperatures around Tg (730 °C) and the color at room temperature was analyzed using optical spectroscopy. Electron spin resonance (ESR) spectroscopy shows that the blue color is associated with the presence of Nb4+ ions, which oxidize to Nb5+ in the clear glasses. Bleaching kinetics were determined from a series of heat treatments below Tg in flowing oxygen, and analyzed with an diffusion model that indicated an activation energy of 150 ± 31 kJ/mol.