From personal experience the color of a man's semen can be very very important in a diagnosis if he is good to sleep with. I had contracted a case of Chlamydia (clap on) when I was still in high school. While I was still trying to hide this from my parents it got worse. One of the first symptoms (besides painful urination) was that my semen turned an awful yellow-green color. So please, watch out for those warning signs of the steady drip drip drip of gonorrhea.