Good points and thoughts by everyone. Especially the point of scored steel looking yellow, which is something that crossed my mind last night, too!
Now, I talked to a few other people who have the same car, and they said they did not get any flakes when they changed the rear differential fluid.
When I changed the tranny fluid, I used a magnetic probe and found fine powder and small silver flakes that looked like stainless steel. The tranny had been rebuilt by Audi at just under 30k miles, but has had a noisy first gear for the past 66k miles.
I didn't stick a magent in the used rear differential fluid, but I can do that today, since I haven't recycled it yet. Let's see if I can find any yellow metal that's magnetic!
PS: I was told my tranny (don't know about the rear differential) contained a few big magnets, but if that's true, how come not all steel flakes stuck to those magnets?