Detrimental auras are auras that affect the enemy in a negative manner.
As for which color is best, I could not say. I'm a Paladin, I focus on the other side of auras

But I tend to use white, green and blue. Grey is also on the Paladin side, and I am not sure about red as it increases direct damage from casting, but that is more of an effect that boosts the player than hurts the enemy.
Just look at the mouseover descriptions for the auras (found in the pet shop) and see if it does something to the player/group or the enemy.