What Beko said.
You can buff them to a degree, but you must buff all of it and its more labour intensive to do than the machine is worth.
We had a DC07 Origin this week that was discoloured somewhat, but the discolouration was so uniform, and it was otherwise a very nice straight machine, we did it anyway. Doubtful anyone will really notice unless standing it next to one that is more silver. And why scrap an otherwise nice machine due to some discolouration? They made a dozen or more shades of grey, silver, iron, titanium, etc. I doubt Joe Soap knows the difference or cares too much as long as he gets a clean machine that works OK. Not everyone are the perfectionists some of us here are.
Cleaning up a grey DC04 though, is next to impossible. With silver machines you can a bit if you have the time, a buffer and some G3. But older grey models, not so much.