Quote Originally Posted by Daralii View Post
It's apparently a pretty simple process to see the color of someone's soul, to the point that it's just part of one of the Alchemist storylines. Ancients can obviously see it without the need of alchemy and can probably better distinguish the hue, but it shouldn't be any more complex than Y'shtola's eyes after she used Flow the first time.
The ALC questline in question was about seeing the colour of someone's aether, which I've always assumed to be relevant to the colour of someone's soul before, but with the new information about souls and Ancients I'm less sure now.

Also it was done through what was apparently the aether version of Luminol, so I'm not entirely certain if it's the "true colour" or if it's the colour as indicated by that specific alchemical concoction.

But Y'shtola does prove that it is very possible to recognize individuals through their aether as seen through aether-sight, as well as how aether shenanigans could make someone's aether unrecognizable.