Video clip is smaller dimensions than project. How to change the color of extra space?
Unfortunately you can't create a project of any size. Larry Jordan posted a workaround, which doesn't solve you're problem.

There was a tutorial that showed how you can have sequence of any size in FCPX (and thus removing black bars), but I can't find it at the moment. (EDIT: I found it! Creative Cow) I remember what you need to do though:


Go to the event library and pick a clip that has the final clips' resolution.
Make a backup copy of said clip (right click, copy&amp;paste)
Right click and choose Open in timeline.
Drag another clip into the timeline. 


This creates what is called a compound clip. Compound clips can be handled like projects, but can have any frame size. A special icon in the Event Library shows you that the clip is actually a Compound Clip.
As long as there are at least two clips in the timeline, the clip's going to be a Compound. 

If you really really really rather work with a project and want to have a custom background to cover up the black bars. I suggest to drag a solid color generator from the effects/generators section underneath your Main Storyline.