Don't forget that you can't mortgage from a color group until you've sold all the improvements from it. Even if some of your nearly-full or full set is mortgaged, the operational unimproved properties still give you 2×/3× rent.
Owning all the deeds of one color gives you a monopoly and you are entitled to triple rents. If you have one less than all of the deeds of one color, then you have majority ownership and are entitled to double rents.
