The Sage of Six Paths had inherited that aspect of power, so under the stimulation of negative emotions, his Byakugan mutated into the Sharingan. Passed down through the Uchiha Clan bloodline, it became what it was now. The resulting ocular power was impure, giving off a cold and sinister feeling.
Chakra was formed from physical energy and spiritual energy, so emotions could directly affect it.
For instance, the chakra of the future's twisted version of Orochimaru gave people a cold and sinister feeling. Naturally, Yang Hao had no need for that kind of impure Sharingan. He just hadn't expected it to turn golden after the impurities were removed.
In the Naruto series, the Rinnegan came in several colors: the purple Rinnegan of the Sage of Six Paths' lineage, Otsutsuki Momoshiki's red and yellow Sharingan, and Otsutsuki Urashiki's blue Rinnegan.
Otsutsuki Urashiki had even opened a yellow Sharingan in the center of his forehead after devouring both of his Rinnegan. Clearly, this golden Rinnegan was the strongest kind.
Yang Hao examined himself in the mirror for quite a while and was very satisfied, though he also felt some regret.
"Too bad this world doesn't have the Purple Qi of Heaven and Earth. Otherwise, using it to assist cultivation would let my eyes evolve faster."
The Purple Qi of Heaven and Earth in the Douluo Plane was a special energy formed by the fusion of the radiance of the sun and moon with the world's vital energy. It could perfectly merge with spiritual power, and that was the essence of cultivating the Purple Demon Eye.
Unfortunately, the Naruto world possessed natural energy, which was vastly different from the world's vital energy of the Douluo Plane. It could not produce the Purple Qi of Heaven and Earth, so the Purple Demon Eye could