A lot of people report that their urine has a dark color or brownish tinge after exercising. This is probably caused by the fact that you have dehydrated yourself while exercising. One of the main reasons for a darkened urine color is dehydration. So be sure that your body is well hydrated before you start your exercises. Keep replacing lost fluids during your exercise session too. In fact, you should be having fluids at least every 20 minutes into your exercise session. Of course, this depends on the level of your physical activity. Re-hydration should prevent your urine from turning brown.