Not that I've tried this, but, what happens if you select the clips, go to the Clip Inspector for one of them, and then make the change while holding down the Control key?  That's the standard SONAR quick-grouping modifier key.
 

Yes this works quite well.  But you would have to do it every time.  As soon as you record a new clip it will have the old background color again.

Here's another question probably asked a thousand times but I can't figure it out.  Is there a way to float the track view undocked?

Matt
As much as I can figure out, there's no way to change the background of clips in a track universally, only the foreground color. (You can change the background of each individual clip in the clip inspector but that doesn't do a ton of good). This is a big dissapointment to me coming from 8.5 but I suppose I will get used to it.

 

Matt
Yes this works quite well. But you would have to do it every time. As soon as you record a new clip it will have the old background color again.

 
This is one of undesirable changes introduced in X3. Those who have been using different colors for clip backgrounds by instrument type must have been disappointed. Strangely, I've only seen 4 threads (including this one) mentioning the issue since the release of the version, though.
 

Matt
My next question is, what good does changing the forground colors do ("1, 11, 21, etc") when those numbers don't seem to actually correspond to the actual tracks? For example, I created my first 10 midi tracks by creating 1 midi track and then duplicating it 9 times. So all 10 of these tracks use the track color for 1, 11, 21, etc. I want those tracks to default to 1, 2, 3, etc without having to individually change (override the default) for every track.

 
I recommend (manually) assigning colors to tracks by instrument type, but if you prefer using the "1, 11, 21, etc" feature, I think it's worth submitting a bug report (Problem Reporter: http://www.cakewalk.com/Support/contact/default.aspx ). And if they confirm it as a bug, come back and post the issue number here so that CakeAlex can list it in the first post of this thread: http://forum.cakewalk.com/Outstanding-Issues-as-of-X3D-UNOFFICIAL-and-general-comments-m2952585.aspx