Simple TCon/SimpleOres incompatibility:
Wearing any armor from SimpleTcon uses Iron/white armor 3D model on the player

Mods used while debugging (1.12.2):
Mantle
Tinkers Construct
Construct's Armory
SimpleCore API
SimpleOres
Simple Tcon
This mod

The color code in conarm-materials.json seems to be completely ignored aswell.

No,
The tool parts are registered by Simple TCon, which adds nothing else.
Apologies for not noting that in the first place.

Does this not happen for the tool parts?

Thanks, I'll take a look at it

Afraid I don't know why this happens and am not entirely sure if this is an issue on my mod or on Construct's Armory side :/

I've found the issue on my end. Will be updating Simple TCon soon.
Had some code leftover from 1.10 that was no longer needed in 1.12, removing it fixed armor render.