A yellow tinge to your semen is not unusual, and no cause for alarm unless it's accompanied by any kind of irritation, stinging or burning. If any of those symptoms are present, you should visit a doctor right away. The yellow colouring can be caused by several factors such dehydration, antibiotics, vitamins (riboflavin), other dietary influences (yellow food colouring) or STI's such as Chlamydia or Gonorrhea.
In response to another user's mention of residual urine, I find that unlikely since it is the function of the bulbourethral or Cowper's gland to help lubricate the urethra, and flush out residual urine or foreign matter before ejaculation. Our bodies really are amazing machines!
To close, I would like to reaffirm you that it is *most likely* nothing to worry about, and I along with most guys here also experience variations in the consistency of our semen from thick and clumpy, to white, to yellow, to thin and runny...
I hope I was able to help