Now that is a very interesting question… took me some time to do a small research. Well, it does not seem to be a common side effect of Duromine or any other Phentermine-containing drug. The reasons for such color change are… starting from pregnancy (no evidence) to dehydration (especially after physical loads) to foods that you eat. Judging by this list, you need to make a pregnancy test (=)) ), drink more, especially while exercising and carefully write down the foods you eat and then check them for the amount of vitamins (B, C) and natural dies that these contain. If you get any unpleasant sensations/pain, you should immediately address a doctor.
Best of luck!