Single use dyes bought with gems
I dont see at all why one would want this, when you can have it permanent.
You can buy dyes from the TP (identified ones) My guildmate sold his black dye yesterday can’t remember how much he got though.
So… buy away.
(if you still wnat to use gems then do what Anet wnats and exchange gems for gold)
To try and answer both questions. Why would you not want it permanently? Perhaps you want to do a guild event of some kind and everyone wants to dress up in all pink uniforms for the screenshots? What if you have a particular set of armour you’d like a specific colour for but it’s not a colour you’ll use in general (say a specific metal colour for trim).
On buying dyes from trading, well you’ve pretty much identified my point. I bet that black dye went for a couple of gold maybe? If I wanted to use black on just a single colour mask, one I may not even keep forever, then I have to pay 2g just to get it. If you set a 75 gem price for all dyes (or even just the rarest dyes since the normal and fine ones are dead cheap) then it’s standard and generally within reach. Plus at about that price you’ve got 4-5 uses before you’ve spent as much as a dye might cost so it’s not a long term alternative, it’s simply quick temporary access.