Team Dye

This one only makes sense on a server. It reads whichever team you have joined and paints the dyed piece to match, then recolors itself the moment you switch, so one dye covers every color instead of buying four separate ones. Solo, or on a server where nobody has picked a team, it does nothing at all and the slot may as well be empty.

The obvious use is PvP, where telling a friend from an enemy at a glance across a big arena is worth more than any particular look. Put it on wings or a cape rather than a small piece of armor, since those are the largest patch of color and the part people pick out first at distance.