Recolouring witcher gear in The Witcher 3
Dyes arrived with Blood and Wine and do one job in The Witcher 3: they change the colour of witcher school armor. They are applied from the inventory rather than at a workbench, and they can be cleared again, so nothing about the choice is permanent.
Only witcher school sets take a dye. Ordinary Witcher 3 armor, relic pieces and the vampire gear keep the colour the game gave them, which makes dyes a finishing touch on a completed set rather than something to use early.
Buying and brewing Witcher 3 dyes
Dyes are alchemy items: each has a formula, and each formula asks for dye ingredients bought from herbalists and merchants in Toussaint. The Witcher 3 alchemy formulas page lists them alongside the potions.
The formulas themselves are sold rather than found, so a single visit to a Beauclair herbalist usually covers the whole palette.