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Also also: vials of prismatic dye are made by brewing or you can buy from other players in the bazaar
Actually, it is Alchemy or Brewing both at a trivial level of 68.
When you hit the Dye tab/button from the inventory screen, it will bring up a new view with a color selection window. You can temporarily change the colors of your armor with no commitment in this view. That way you can mess around with various shades and combinations. I think there is a "save" button at the bottom of the color selection window that will consume the required number of prismatic dyes in your inventory.