Hello,
Normal semen colour is whitish yellow. With increasing age the semen can become more yellowish, less in volume and more thick. The other causes are infrequent ejaculation and infections like infections of the prostrate or STD.
Semen analysis , prostate examination and examination of the prostatic fluid and ultrasound prostate are required to confirm a diagnosis. I suggest you to consult a urologist soon.
I hope it helps. Take care and regards.