Dunno why your white dye didn't work, but the armour might have been white before you applied the dye?
(The game will still use up your dye.)
Like LeBurns said, you'll lose any applied dye after you tinker with your item, so it's best to only use dye after you're sure your armour is for keeps and that you're done modifying it.
In my opinion Larian should fix this. Applied dye should be permanent.
