In this work, we report a quantum chemistry mechanistic study of the hydroxyl (center dot OH) and hydroperoxyl (center dot OOH) radicals initiated oxidation of indigo, within the density functional theory framework. All possible hydrogen abstraction and radical addition reaction pathways have been considered. We find that the reaction between a free indigo molecule and an center dot OH radical occurs mainly through two competing mechanisms: H-abstraction from an NH site and center dot OH addition to the central C=C double bond. Although the latter is favored, both channels occur, the indigo chromophore group structure is modified, and thus the color is changed. This mechanism adequately accounts for the loss of chromophore in urban air, including indoor air such as in museums and in urban areas. Regarding the reactivity of indigo toward center dot OOH radicals, only center dot OOH addition to the central double bond is thermodynamically feasible. The corresponding transition state free energy value is about 10 kcal/mol larger than the one for the center dot OH initiated oxidation. Therefore, even considering that the center dot OOH concentration is considerably larger than the one of center dot OH, this reaction is not expected to contribute significantly to indigo oxidation under atmospheric conditions.