This What is Crowley question has been a pretty interesting battle on the net. We all get this, he is the King of Hell and he is a demon. However our man Crowley does not have the typical trademark of black eyes, like the other Demons, he has the red eyes of crossroad Demons. Also when he did switch to a different body, Kevin Trans mother.. His smoke was red.
A Supernatural fan states "Crowley was the king of crossroad demons before the king of hell, crossroad demons have red eyes. It's possible his red smoke therefore is associated with the status and power of that branch of demon. Something unique that causes the color change, perhaps the power that comes with the deal making reality bending powers of the deals. There's also he idea that he excels in magic, perhaps he did some sort of spell or self modifications to make himself more powerful. Whatever the case we can only hope that with Crowley becoming more of a regular they might find time to explain it. Otherwise we must just assume it's something like the yellow or white eyes, something unique to show that the demon is in a different league of the others."
Mark Shepherd the actor who protrays Crowley stated on tweeter "who said I am a demon" to one of his followers when she questioned the color of his smoke. My personal opinion is that Crowley is the lead crossroad demon, who got a promotion to be the King of Hell since Lilith was killed and Lucifer, the rightful ruler of Hell was tied into some other stuff. So I think his smoke is red showing us the view that he is just not a crossroads demon, He is bad ass with an upgrade. He still makes deals like the crossroads demons do but he is much more powerful then your everyday, run of the mill demon. He is just a peg below Abaddon, Cain and Lilith I believe. If the demons follows some type of hierarchy.