A combined theoretical and experimental chemical analysis has been conducted to unravel the mechanism, underlying the color change of yellow 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) to green upon UV irradiation. There is a strong evidence to show, for the first time, that the process is photochemical in nature and due to the formation of the mono nitroso derivative. We have identified a chemical synthesis by which this derivative compound can be produced in the laboratory, thus allowing for direct testing and determination of its chemical and physical properties. Calculations also show only a slight decrease in the sensitivity and performance of the irradiated materials, attributed to the formation of this previously unidentified species.