The luminance signal needs to have full resolution (i.e., unbinned), but colour data can be lower resolution and theoretically the eye will take the detail from the luminance.
Up until recently I didn't have the best luck binning colour 2x2, but I also didn't try all that hard. I recently bought a Trius color camera and have used that data to combine with high quality luminance. In some ways one could argue that the colour data from the Trius-c is not even as good as a mono Trius binned 2x2 given that out of an array of 4 pix there is only 1 red, 1 blue and 2 greens. However, I think this result looks pretty good:
http://www.astrobin.com/full/232099/0/
By the way, CCDStack has no problem at all combining binned images with un-binned images. It does so automatically.
Peter