Ok so it's pretty self explanatory why Nashandra's soul has a "dark" appearance, (she is Manus, and Manus' soul was almost identical) but why do the Throne Watcher, the Throne Defender, and Vestaldt's have a similar appearance? Weren't they working against the queen?
The only time she shows herself to you is after you have reclaimed the King's Ring, (killed Vestaldt) and cleared the Throne of Want, (defeated the Throne duo). So if they were corrupted by the dark, would they not be working to help Nashandra?
Artorias (in DkS1) was corrupted by the dark, and his soul has a similar appearance, but even he had the sense to attempt to ward you off, in hopes that you would not fall to the Dark, before he was completely overtaken. Source: Cut Artorias Dialogue
Best Answer
Since the lore of the Souls series is intentionally left open to interpretation, this is mostly speculation.
Nashandra was born from fragments of Manus, she is pure dark.
The Throne Watcher and The Throne Defender did just that, they watched and defended the throne, they serve Nashandra.
As for Velstadt, opinions vary, but a theory I like is that he isn't actually protecting Vendrick, but making sure he can't get out.
As you can see in the game, Vendrick has completely hollowed, which implies that he couldn't have gotten to any souls.
Which in turn means Velstadt didn't let him out to get souls.
So the way I see it is that just like Artorias, they've all been corrupted.