Kristin Cavallari and her boyfriend ... a white dress that matched her boyfriend's classic white t-shirt and denim jeans. As for Jason, he chose dark pants and a white button-down flannel.

Kristin Cavallari is ready to prove "she's country ... But as the guys lip-synced to the track, Cavallari appeared behind Estes in denim jeans and a shirt that read, "This Ain't My First Coors ...