I agree with Chikky-I am sure you could find a way to ask without her guessing what you're up to! I bet you could even offer to help "set up" the new house; since her new DH will be away I'm sure she'd appreciate the company and help! Then you could get an inside look so to speak and have a better idea of what color to go with.
If nothing else, go with a neutral, like Jan said-a nice cream or warm soft brown-silvery gray even? I think checking out her favorite wardrobe colors is a good idea, too.
Boy, I'm a big help
Well anyway, I am sure no matter what yarn you use, she (and he) will LOVE it!