Q: What does a draconic aura look like?
A: The rules are silent on this issue. Neither the Player's Handbook II (where the dragon shaman and his draconic auras debuted) nor Dragon Magic (which expanded the feature to additional characters) describes a visual effect, so it seems that the aura is an invisible effect.
However, if it suits your game for the draconic aura to have a visible effect, that's just fine. Make sure that the aura's appearance doesn't have any game effect (for instance, it shouldn't illuminate an area, nor should it interfere with a character's ability to hide).