Probably everbody hates that you cannot influence the mana color of an item, that you upgrade from uncoomon to rare. However, I can understand why the developers don’t change it.
An idea: Give an ally the ability to choose an item and the ally switches the item color into another RANDOM color.
Same thing could be provided for item attributes. The ally level defines the maximum level the item may have. This allows you to upgrade uncommon items to rare and then keep on transforming it until it has the desired color.