When men ejaculate, the purpose of their semen is probably the last thing on their minds, but it can serve a very important role in spotting changes in your well being. Usually, semen is a white or off white fluid that within about 30 minutes turns runny and clear. Any changes in colour or smell could be a sign of something more serious. Semen will usually have a chlorine type smell, but a strong foul smelling ejaculation could be a sign of infection. Yellow or green tinged semen could also indicate infections and red or brown semen can highlight prostate problems. The amount of semen produced varies from man to man. However, recurring low levels of watery semen could indicate a condition called retrograde ejaculation and though not harmful, it can cause fertility problems. For more details check here and watch the video