The armor model when worn on players/entities cannot be remodeled in vanilla Java edition, only retextured. Pumpkins and other items set on a player's head can be remodeled for custom hats, but won't display if set on feet/legs/torso.
(Any item can be put on the players head via the /item replace command (aka /replaceitem before 1.18), or ofc via mod or plugin methods). Furthermore, with this pack you can give a large set of specific leather armor colors a unique texture without interfering with the colors available in a normal survival situation: https://github.com/Ancientkingg/fancyPants
With Optifine it is possible to add texture variants on any armor pieces depending on the name, damage and other nbt, but remodelling is also not possible with it.
hushed helm
zenith topaz