I think you're making an assumption. Her mother may know what her favorite colors are but not the colors she and fiancee have chosen for their abode. Most marriage couples I know have a registry, maybe at Bed, Bath and Beyond or whatever store is in their area. That would include colors to use in their house. It would also give you some idea of what type linens they plan to use. And what style the two would like. That might give you more accurate information and a lot more of it.
Unless I was a very close friend I wouldn't make a bed quilt for them. I'd do something a little less permanent, say a throw.