Hi Lee,
As you have already hinted, with green filter you will have significantly higher background signal, followed by blue, and red should have the lowest background signal for the same exposures. It makes sense to me that with brighter background you need to expose for longer, while with perfectly black background you will be only overcoming noise from bias, thus exposures can be shorter
EDIT:
(From
http://www.stanmooreastro.com/eXtreme.htm )
(...) Thus bright objects are not much harmed by light polluted skies.
(...)for dim objects, the time and sky terms nearly operate in tandem. For example, a 2x brighter sky requires almost 2x more exp time to reach a similar dim object S/N. A typical semi-urban/suburban sky will have a brightness of approximately 17 mag/arcsec^2 and a good rural sky is about mag 20. That means that a dim object will require approximately 16x more exposure time under the bright sky to equal the S/N of the dark sky.