I think you need to study a bit more how rendering works, especially in webGL.
Imagine that in real life you take a mirror in a completely dark room, what color will the mirror be? Answer: Black
It’s not enough to export the mesh and materials to gltf, and expect it to look like a photo, even if in blender it looks ok, the default shading view in blender will have a HDRI applied.
Look into how to add a HDRI to your gltf viewer, and the effect will dramatically improve.
It would be best if you could actually post some images so that we can give you some more accurate help.