I'm sure there is a better song out there than this, but I've always considered "Sunflower" by Mandy K to be a Brulian song. "Indigo" by Far Beyond Frail is also a good song to describe Brooke's emotions when Julian leaves and also her struggle to tell him she loves him.
I think the song that best describes Brulian is "You Found Me" by The Fray. I love this song and I love Brulian. They actually played it when Brulian we looking at Julian's high school year book and reading the description of the "Brooke character for Lucas's movie. After hearing it during this scene, I feel in love with this song. I think the song and Brulian are perfect!