This may be of help to you.

http://www.teenhealthfx.com/answers/Sexual ity/1233.html

A friend of mine told me once that if a man goes too long without ejaculating, his semen becomes "dirty". He claimed it becomes thick and changes color a bit (I'm not sure what color). How often do you masturbate?

this depends also of your raze I am a mix of latino and white and my semen always is yellow, my doctor told me this is normal for me. So maybe this is your
"problem" which I think is not a problem at all.