Why?
I mean I rarely stare in bewilderment at my screen but why the flying forge would being able to dye Dropped armor be a bad thing? I can see Materia needing to be "Balanced" In some way, but seriously what would making them dyable do except make a large portion of players happier and maybe stay the wave of Clones you see at level 50?
Half of this game is Vanity like most MMOs(Pets, Mounts, etc), So I simply cannot understand why dyes should not be allowed on high end gear. I can see the argument for AF/Relic gear holding water what with em being FF history etc... But the other stuff... Should absolutely at the least be dyeable.
I like Darklight Armor, It looks nice, But I'd personally prefer it in a Dark Purple or a Red. I have those options on my Grandma Robes, but the few pieces of gear that actually look good and I'd care about dye-ing are undye-able... and that just sad. Nothing negative can come from allowing Dropped/Bought Green Tier+ Armors to be dyed.