This is a truly baffling one for me, and following on from a previous issue that I believed had been fixed.

For some reason, the grass.properties in my custom pack, when loaded, obstructs the appearance of the vanilla Shield/Trident GUI icons. When grass.properties is removed, this is no longer the case.

Does Colormatic require a different format in order to avoid this conflict? I'm puzzled as to why the registration of a file that has no relation to items is then obscuring them.
grass.txt
No further .json issues are present on the Shield, and the Trident has no .json data in the pack.