When I was in college I played on my university's soccer team and there was one season when I suffered from a yellowish sperm. We started training in the summertime and it was brutally hot that year. I thought that I was drinking enough water, the coaches pushed it onto us at every break, but apparently it was not enough.
I began to notice a change in my sperm when I was by myself one night and it really freaked me out. You get pretty used to your own body, especially in regards to sex, so when something changes it can be really jarring. After fretting over it for about a week I finally went to a doctor and sheepishly asked him what was going on.
He told me that I was dehydrated and actually hooked me up to an IV right there in his office. Afterwards I made sure to drink even more water during soccer practice. I would also drink a ton of it before and after. My sperm changed back to its normal color pretty quickly and I actually started to play better. I guess that I was more dehydrated than I realized.