Light

"Do you know how the light spectrum and colour works, dear disciple?"

He nodded. "Are you saying the colour is different because of the spectrum of light?"

"That too," she agreed, though there was something more to it as well. She seemed to debate over the explanation for several moments before opening her mouth. "I don't know if my explanation will make it more difficult for you to understand, and anyway, the first thing you must know is that light is not a path unique or exclusive to celestials."

He frowned but nodded.

"Light is but the fastest medium to exert the essence of power," Ariha said sagely, as light sparked within his mind. "Your void essence mostly emits in gaseous and light form, and what colour does it take shape in?"

"Dark purple," he mumbled.

She nodded. "On a side note, you exert a lot of void essence, but you waste a lot of it... Like most of the gaseous forms of void essence are a waste."