Personally (and these are my own thought and opinions) I would like to see defense teams have some kind of incentive to vary each week at least, and not just be grief-of-the-month/week/day teams. While making them adhere to the War’s colour would possibly do this, it would likely make fights easier, and I don’t think it would be addressing the cause of the issue.
I think the underlying reason for the “defense must use the day’s colour” argument is more to do with the tendency of players to use the grief-iest team possible on defense in the hopes of knocking a few points off of the attacker. I agree that the difficulty that GW brought is good and should stay, but it needs to come in a form of something other than “who can make the grief-iest defense team”. Maybe a rework of the scoring system would work better?