Most of the time, if Sam were just asserting his ownership of Castiel, it would still be the blue collar. Tonight, though, Sam doesn't think Castiel needs the blue one. If he wants punishment, then he will get it in all of the forms Sam has for it.

He reaches around, buckling the collar behind Castiel's neck and double-checking that it's not too tight. Wouldn't do to choke Castiel as well, after all. Then he takes hold of the ring at the front of the collar. "Stand," he tells Castiel, even as he starts to move himself. So that Castiel HAS to stand with him.