Only two types of packets have mandated colors: arrows and gas globes. And the thing they both have in common is that they are both physical objects.
The idea behind the colors is that you should be able to immediately identify if someone is holding an arrow or a gas globe, because they are distinct physical objects that should be identifiable if we weren't using phys reps for safety reasons. Aura doesn't have a color (other than not blue or orange) because all energy looks similar, regardless of what is creating it. So, the energy of an unformed elemental energy attack is indistinguishable from the energy of an unformed Life spell or of an unformed arcane fear blast. Those effects only become distinguishable when formed, which is represented by the verbal for the effect.
-MS