I'll let you in one a little secret I learned from Vicarious Visions recently. That purple is Rattle Shake's NEW Soul Gem =)
He basically become encased in skin of poison, that poison takes damage that enemies deal to you for a while until it comes off. Then he is naked and restores his regular skin back.
-wdrumz
I thought his soul gem was revealed back in one of skylanderkids' videos. But I like how the Soul Gem will play out.