Missing Materials
Sometimes your model may not be loaded with the original colors or textures.
There are a few reasons this might happen:
-If your model uses textures, these textures must be located at the same folder as the model, or inside the model Zip file in case you're loading your model from Zip files.
Model textures can be embedded in the model itself depending on your exporting options. glTF2 and FBX files accepts this feature.
-If your model doesn't use textures, make sure the resource files your model uses are located at the same folder as the model, or inside the model Zip file in case you're loading your model from Zip files.
glTF2 files sometimes comes with ".bin" resource files, and OBJ (Wavefront) files sometimes comes with ".mtl" (material library) files.