I have had a brand new Audi A3 Sportback for about three weeks. It is kept outside and has only done about 250 miles. From day 1, the brake discs have shown signs of yellowy discolouration, such that the front discs now appear almost gold in colour. I assume that this is an oxidation/rust issue, and if so, is it normal and what (if anything) should I be doing about it? Thanks.