In order to make the level 50 armor dyeable (the one you get from completing your jobquest) you'll need to complete the quest "Simply to Dye for".
Then you'll be able to exchange your artifact gear and a specific alchemist craft ("[artifact name] armor augmentation - this one for example for paladin: https://na.finalfantasyxiv.com/lodes...e/b27420ca352/ ) in Mor Dhona to get a dyeable version of the set. Note that this version cant be stored in the armoire any more. This only works for the stuff you get from the quests, as far as I recall there is no way to make the ilvl90-gear dyeable.