Sketchfab supports a maximum of 100 materials per uploaded model. If we detect more than 100 materials during processing, we attempt to merge identical materials. If there are still more than 100 materials after that, we merge everything together into a single material, which is why you might see your model with an off-white material color.
In order to upload your model with your materials, you'll want to compress or consolidate your total materials and re-upload. For more details, see Materials and Textures.