I have no idea if the following is correct; but I was once told by someone who appeared to know what they were talking about saying that meteors/meteorites generally burn with a white light whilst space junk quite often has a colour (mainly green but occasionally red, yellow or purple) to it because of the various alloys used in construction of whatever it is that is re-entering the atmosphere.
I am quite sure that there is a ppruner who could confirm, deny or expand on this with some authority.
