Board Thread:General Discussion/@comment-43877988-20191205041833/@comment-27745396-20191206175909

Oh, right. I remembered Blake's and Yang's aura colours were other way around. Some BB fans like to explain it with Blake and Yang being "soulmates". Something about their auras matching with other's eye colour, I think. Purple is still part of her colour palette so it didn't come completely nowhere

In other words, the colour of aura is most likely chosen just to fit with the character's colour palette, but different/bright enough from their clothes so it to be easy to see. Though, I think they might end up changing Mercury's aura be closer to grey to separate it better from Jaune's.