Good guys wear white because they don't expect to loose.
Unfortunately they often do, and when they do they look like martyrs. Honestly I can not see how this inspires courage to the troops they lead. Even though we fight with magic, more often then not in a real dual or fight we magic folk tend to bleed. And while I can see how heroic this might seem to some it would not instill me with the idea that my leader was protecting me. If he was bleeding profusely and blood dramatically soaked his shirt I would not think that I was safe.
It seems rather weepy to me as well. Because then they insist on the their death bed that they be buried in that bloody shirt. So everyone gathers round and looks upon their reconstructed body the blood still looking fresh thanks to a coroner's charm. And they look at this dead man, crying over the irony or the shame.
But they chose it. These white shirts are not charmed for protection, they aren't even that flattering. They look very heroic.
The other problem with white shirts is that show up in the dark. I've been out at night on prefect duty and ten to one most boys are too lazy to change out of uniform before they go neck in the woods with their girls. They are easy to spot. Even running and hiding doesn't do much. A flash of white in the small beam of light a lumos spell casts is easy to see.
Black shirts on the other hand fade in to the dark, they blend in easily against the dark greens and browns of a forest, the wide expanses of a field.
Blood doesn't show up on a black shirt. A leader in black can drop dead of blood loss and no one know why until later when the battle is won. This does mean less room for heroics. But evil doesn't have time for such nonsense. The individual matters less for the dark side, the end result it the goal. And anyway if the good guys win you're body will be destroyed as an example to all those who might follow in your evil footsteps. And in all likely if the evil people win and you're dead those who were under you will be lining up to piss on your grave.
Being evil also means that your black shirt is well protected. Most likely no matter how nice it looks it will most likely be lined with dragon hide and spelled against all but the unforgivable. This means that a lot of the aurors have been lying for a long time about their innocents. They don't really have a choice, or what separates them from the evil ones?
Bad Guys wear black because they don't expect to win.
But I will wear black at the last battle, as I charge in alongside Harry Potter. Because really, I'm a blonde and we look better in black.
Disclaimer: Own Nada OH! I own the Coroner's Charm!
Author's notes: Inspired by a western. Gotta love gunslingers.