I would like to have the ability to paint my units Armor. Yes, including leather armor, scale mail, and Iron Plate.
Forgetting, for the moment, the practicality of such a thing, it could add to the uniqueness of the units, as well as working together with the Name and the Items/Weapons to create customized and stylized ... perhaps recognizable or distinguishable units.
Namely, the ability to have black, green, brown, or blue leather, and Red plate-armor. (or Blue plate, or Green, or Black).
Now, perhaps this would not be allowed initially, but unlocked with a multipurpose civilization tech. Something like Tan-dyes for leather "painting" that also unlocks the Tailor building. Or Glazing for "painting" metallic plate-armor, which allows for the creation of a Potters Guild.
For instance, I might wish for a Regiment of "Blood Angels" to be wearing Red suits of Iron-Plate armor, or for a company of stealthy "ninjas" to be wearing solid black leather.
personally, its just another level of aesthetic customization for our units that I would definitely love. They had something similar in Dawn of War, although it was a one-time paint customizer for the entire army. Elemental currently has such a color-customizer, although only for Cloth-style clothing (namely civilians).