All I found was the Dyer addon that lets you preview dyes on costumes (which is awesome). I was wondering if there was something like that to preview dyes that haven't been unlocked yet?
ZOS should have implemented this years ago
Not that I've ever found. There's also, presently, no dye unlock option for the PTS, and most sites that list dyes only display the monocolor thumbnail, which doesn't help when you're looking at rare, iridescent, dyes.
If you've got a dye in mind, a lot of people can probably tell you what it looks like, but you can't see that in game, so far as I know.