I cannot change the appearance of a particular face of a part in the Assembly. It does in the particular part file but does not shows up on the assembly. It's only for one part of the assembly all other parts are okay.
you may have an appearance applied to the part in the assembly that is overriding the part appearance.
In the attached picture of an assembly, the bottom triangles in the appearance column are from the part file, the top triangle is an appearance applied in the assembly that overrides the part appearance.