The Equipment Cube are not colored correctly unless sodium mod is uninstalled.
version: minecraft 1.20

pls provied mod list

My shader mods:
iris-mc1.20-1.6.4
sodium-fabric-mc1.20-0.4.10+build.27
indium-1.0.18mc1.20

!! and when I set Video setting /quality /biome blend into 0 ,Equipment Cube show correctly(I try all setting to find this :))

Yes, we don't support sodium biome blend, and we don't know how to support it. a bigger problem is that shader mod/pack will cause the cube color to confusion, so never use the shader mod/pack when make skin.