Dye lv 50 Relic

[Vespar]

Are you talking about dying your relic weapon or dying your level 50 job-specific armor?
If armor, its still kind of a pain. The reason being that you must make attire augmentations and trade your old lvl 50 piece along with the augmentation item(s), to an npc in Mor Dhona.
That npc will then give you back the augmented piece. Which is now dyeable, however, its no longer storable.

Crafting the augmentations is the annoying part. While the augmentations vary depending on the job, the materials necessary can still be a bit pricey to get. Some of them use things like Vanya silks, Terminus Putty, etc to craft. Which is why they can be pricey.
You can check what and how many augmentations you need for a specific piece at the NPC Kakalan in Mor Dhona @ 22x 5y.

As for relic weapons, dying them means reaching the last stage of the 2.0 relic quest line (Zeta). Then and only then does the weapon become dyeable.