What are the transmog rarity grades?

Gold is the grade you are rolling for, and rerolls only land on it some of the time. Blue sits at the bottom, purple in the middle, and the yellow grade above purple is the one also called legendary.

Since a gold result is not guaranteed, protect a good roll before you try again. You can save a Transmogrify result as a preset before rolling, and the history keeps up to 10 previous rolls, so a purple you liked is not lost the moment you reroll for gold.

The system itself is narrow. Only the Iris line, Irisalis included, can be transmogrified, so these grades never apply to the rest of your team. Rolling uses two appearance gacha materials, a yellow one and a purple one: Radiant Dreamshadow and Shimmering Driftshadow.