Make it to where dye barrels can't dye plot items.
Since most things flagged as "plot" aren't actually that, I would disagree, as it is more hassle finding a DM to allow you to dye something like "MUSHROOM LEATHERS" that have no bearing on the game world than it is to allow this.
I'd rather have it so that all items are able to be dyed. It'd make more sense, as black helmet should be able to have its paint grinded off, and plain steel helmets should be able to be painted to a certain color. Same as banded mail, suits of full-plates, leathers, etc.
It saturates the game world with everyone having their own customized attire. Which is terrible. Perhaps then if it's a plot item, when you go to dye it there's a chance you destroy it. Since when you're tampering with highly magical artifacts, restructuring their make up - it'd make sense every now and then you make a boo boo and render the entire piece useless. Hell, a good use for the tailor skill.
Some items I don't understand why it cannot be altered. Yet some make sense, since the DMs want them to look a certain way. I think items like H'bala's Fullplate of Withering Terror should likely stay how the DMs designed it.
Dye barrels do not change the appearance of the armor, only the colors.
For ease of convenience, they do dye armor marked as plot.
They are used to dye armors, cloaks and helmets to fit the colors of a faction/association, making its members more easily distinguishable.
There's no compelling arguments here for adding restrictions to how it works, so will close this suggestion thread.