To my best understanding, a rupture occurring during ejaculation can cause sperm to be discolored, but usually blood tinged. However, I have heard that symptoms in a man of an STD can range from having yellowish or greenish colored sperm, usually the cause of an infection of some sort. Therefore my best advice is to have him get tested for Gonorrhea and Chlamydia. In the meantime, you should also be tested for the same, and be abstinent until you get yours or his results. Good luck with that....