Black (and other dark colors) - Both of them are seen wearing dark colors frequently. Black also symbolizes Jade's personality and why Beck loves her.
Heart - It symbolizes their love for each other
Rottweiler, because this is the dog Jade tried to get Beck when she broke up with him in Jade Dumps Beck.
143, because this is the number on Beck's RV when Jade and Beck get back together. According to one of Dan Shneider's blogs, the 1 is for I, the 4 is for LOVE and the 3 is for YOU.