If I understand what you are wanting, then you need to proceed as follows:
Put your background on Video layer 1 and colour correct it as required.
Go to File/New/Color Matte and set this to your fill colour, in this case white (but it could be anything you want).
Add this Color Matte to your Video layer 2.
Add your green screen footage to Video layer 3 and add the Ultra Key effect to key out your footage.
Then go to your Color Matte layer and add a Track Matte Key.
in the Effect Controls for the Track Matte Key, choose Video 3 as the Matte layer.
That should give you the effect you want.

Hope that helps.
Simon Ubsdell
tokyo productions
hawaiki