Aura Sphere has a fixed colour for a fixed Pokemon. For example, it has appeared many times in the anime:
Togekiss used Aura Sphere in the Diamond and Pearl anime, episode: Dawn Of A Royal Day!
It opened it's wings, and unleashed a blue sphere. Aura Sphere
Lucario used Aura Sphere in a few episodes, such as: Lucario, and the Mystery Of Mew
He too unleashed a blue sphere. Aura Sphere too
But for Palkia and Giratina, they used Aura Sphere aswell, but different colours. Pink and Gold
So overall, In the anime, Aura Sphere's colour depends on what type of Pokemon uses it.
But in Pokemon Games, I'm 99% sure that they are all blue when used in a game.