should be able to select your yellow helmet, the transmog of the unique should be in the pictures, usually at the bottom of all the others, and if I remember right, it's 50K instead of 500 gp, I don't remember there being an item required to do it, but if there is, maybe that's why it isn't working?
WD Season 8 https://www.diabloprogress.com/hero/finiar-1655/Kildare/84509816
Monk season 7 http://www.diabloprogress.com/hero/finiar-1655/MojoJoJo/42225505
DH season 6 http://www.diabloprogress.com/hero/finiar-1655/DeadShot/75655606
Angry Chicken http://www.diabloprogress.com/hero/finiar-1655/WhoDoVooDoo/68187610
What? Me worry?