You have two currencies Glimmering Driftshadow and the gold  Radiant Dream Shadow.

The system defaults to Radiant Dream Shadow so make sure you are clicking the " Use Exclusively Glimmering Driftshadow"

Unlike gacha systems we are used to, the  transmog gacha doesnt let you keep your roll unless you save it to a preset, which you only have 3 of.

  
You have to unlock each section of transmog by using Glimmering driftshadow and the next part does not unlock until the meter is filled. So your first few pulls you are only rolling for the color. 

Once you unlock the next section both are RNG rolled, so if you get two gold transmog , you'll have to make sure you have a preset slot open to save it. You will have to delete one if you don't. 

Unless the slot is locked you will be RNG rolling for every cosmetic piece every time.

Radiants can also be used to roll, but its best to use them to "Lock" the slot from rerolling.  

In short, if you want a particular cosmetic effect, you have to roll to unlock the slot then roll for a chance to get the the cosmetic and then save to a preset outfit. 

Warning!  
  
 if that preset is deleted and you didnt lock your cosmetic you will lose that cosmetic effect and have to roll for it again. There is no way of getting it back unless you reroll for it.

What you should Know about Irisalis Paid Skin