Dehydration is the most common cause of dark yellow urine. Darker the urine, lesser is your fluid intake. However, observe the color closely. The color due to dehydration ranges from moderately yellow to dark yellow. However, if it appears brown instead of dark yellow then it may not be dehydration but liver disease. To confirm dehydration, your doctor is likely to recommend specific gravity test. This test compares the density of urine to that of water.