What currencies does transmog use?

One is bought with real money and the other with free Glimmer, and that split decides how far you can take a look. Radiant Dreamshadow is the paid material: one costs 200 Lumin Crystals in the item shop. The free one costs 200 Glimmer each from the Glimmer tab of the shop, and you will also see it called Glimmering Drift Shadow.

Both go into rolling and unlocking Irisalis transmogrifications, which change the colours of your Irisalis. Radiant Dreamshadow is also the material for changing the appearance of legendary Aniimo.

Where the free material stops working

The free material only takes transmog as far as the third step, the headgear slot. Anything past that needs Radiant Dreamshadow.

Locking a slot has the same effect. Once you lock any transmog slot, free material can no longer be used on that reroll track, and every roll on it costs Radiant Dreamshadow.

Check the free currency toggle before every roll. The transmogrification menu can uncheck it by itself, and the roll then spends your paid material instead.

Is it worth spending Glimmer here

At 200 Glimmer a roll, transmog eats into a currency that gets harder to come by. Free Glimmer income drops sharply once the one-time sources run out, notably past roughly level 58 to 60, and Glimmer has other uses that may matter more than cosmetic rolls, such as Capability Fruit at 1,000 Glimmer. Daily activities pay 30 Glimmer, so a single roll costs more than six days of that reward.

Not settled: The free material appears under two names, Shimmering Driftshadow and Glimmering Drift Shadow. Both refer to the same purple material bought with Glimmer.