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.