I have the same color combo on a 3 year old RX330. The carpet is mint .. no signs of any discoloration. Only thing I can think of is that somehow this is chemical related and possibly brought in from outside on your shoes (ie rainy days).. Maybe track your habits, where you walk, what you walk in (ie rain water etc), and if you wear the same shoes for work etc. Other than that, you seem to have a tough one to figure here.