What you were probably seeing was the metal tarnishing/corroding from being in the mouth for an extended period of time. New braces started out big, bright and shiny, towards the end of the treatment they looked dull and dark. This was would be due to a few reasons: 80s braces were bigger  and stayed in longer,  but made with a stainless steel alloy that was not as resistant to the acids in the saliva. Modern metal braces are small,  made out of tougher steel, and work faster. These days, most orthodontists push hygiene a lot more than before, and patients have better tools available to them like the sonicare brushes. Also, some of the alloys of the improved wires may have been reacting with the brackets, as the relationship between the different metals in the mouth had not been fully tested. Finally, some wire/ligature/chain combinations seem to add to this effect, depending on their color/material and/or if they have been stained from use.