In that game you can change the actual model of the gear you are wearing to look like that of something you want to wear.
So in Final Fantasy terms, if you take PLD AF to the NPC as a DRK you can apply DRK stats to it and then wear it with pride as a DRK.

Yes I know this is a fantasy game, but in trying to accommodate to both players that want color freedom and those that want more stat customization, enchantments or runes are the best option as neither of the two are really visible, and from what I have read that is the real argument here.
Stat customization really has nothing to do with the argument, it's the class identity that is an end and having different stats is simply means to achieve that end. Just downright restricting an armor to one class is a means to the same end.