How to import obj file in blender ?

Hi, I'm not sure if you still need to know this, but the reason you are having this problem is because you are trying to import an OBJ file into Blender, and not all interchange formats support materials. In your case, the only file type that would let you import materials would be .BLEND, because it is the native file type of Blender. Any other file type will just import a bunch of gray materials.