I have no idea what can you find in this library, but I can note that just the definition of "predominant color" will appear way more difficult than you probably think. Well, you can calculate the color histograms, find some modes (or not; http://en.wikipedia.org/wiki/Mode_%28statistics%29
]); and — what will be "predominant"? Besides, the term "background" is very relative, and certainly can be defined not from all scenes, even posters.
Those considerations let me suggest that such function hardly can exist in any non-nonsense library, because it's hard to imagine what should it do.
If I'm not right and miss something here, I would be interested to know what such thing could be.