The presence of bacteria in urine, such as with a urinary tract infection (UTI), can alter the look and odor of urine. When the urinary tract is infected, the urine may have a foul-smelling odor and appear murky or crimson. The color of the urine is due to the presence of bacteria inside the body that produces red blood cells. Urine has several components including water, salts, proteins, glucose, and other substances. The color of urine is determined by its content of red blood cells. If there are no red blood cells in the urine then it is clear or white. If there are many red blood cells but not all come out when you shake the bottle then it is pale yellow. If most of the red blood cells do come out when you shake the bottle then it is dark red. The color of the urine is important because it can signal an underlying medical problem. For example, if the urine is black then this might indicate that you have bladder cancer. On the other hand, if the urine is very bright pink or red then this could be a sign of serious kidney problems. A smell that comes from within is usually not harmful, but if the smell is strong or unpleasant then you should try to find out what is causing it. You may want to keep an eye out for warning signs of more serious health problems.