Proteins and amino acids that contain phenyl rings form a yellow colored compound when treated with concentrated nitric acid. The yellowstains on the skin are caused due to the reaction of nitric acid with protein keratin present in the skin. This reaction is called xanthoproteic reaction.
Nitric acid takes yellowish brown colour due to the presence of dissolved nitrogen dioxide. Nitric acid slowly decomposes even at room temperature, specially in the presence of sunlight. Liberated nitrogen dioxide dissolves in the acid and gives a yellowcolour
Therefore NO2 or nitrogen dioxide is responsible for this change in colour