No, it's not possible to have clip background color follow bus assignment. I found that you can import default background colors exported from X2, which will assign colors by track number, but as soon as you start re-assigning foreground colors, that gets out of whack, and it doesn't help with you goal of matching the bus color. The best you can do for now is select and change all existing clips' background color as a group; this is pretty easy to do, but does not address subsequent recordings, which will use the default color for the track.