....#4. The "stuff" is not blue/red/straw in and of itself. Rather, various thicknesses of coating cause reflectance of various wave lengths and we see them as differing colors. Blued/blacked metal appears dark because the oxide coating absorbs most light and reflects little. Note, the prismatic colors we see from a puddle of water with a bit of oil on top is caused by varying thickness of the oil film....
This is a tough one. It's been noted by Doc Gaddy and others that the colors reflected off an oil film change as the viewing angle changes, but color perception of case colors doesn't. It might also be supposed that fading of case colors may not come from wear, if wear can be assumed to be a decrease, change, in thickness of the reflective layer.
