Reality Converter convert GLB to USDZ

Without seeing exactly what environment you're testing your files in/with, it could be a multitude of things:

• The simplest explanation I can think of is the environment lighting could simply be different/darker.
• The color space or texture maps were affected (not likely though).
• The material shaders could've been affected during conversion. Texture maps have a multiplied factor from 0-1, or black to white. You can adjust this in some gltf editors. It could be possible that the conversion is multiplying the factor on the color maps with a value less than 1 (so a grey or darker color).