Human semen is a composition of fluids. The majority of semen, 65%, is composed of seminal plasma and mucus. 10% of semen is composed of sperm produced in the male testes and the remaining 25% of semen is composed of prostatic fluids.
Normal semen is milky white but a number of factors can affect the texture, color, and odor of semen such as medication, diet, and diseases. A yellow tinge to semen is not uncommon especially if one has not ejaculated for some time. A brown tinge to semen can indicate old blood. This can be secondary to pelvic floor trauma/injury, prostatitis (inflammation of the prostate), orchitis (inflammation of the testicle), or malignancy.
If your semen was brown only on one isolated occasion, it just could just a transient phase. However, if you are concerned or your semen has been brown from some time, is becoming progressively browner, or you have noticed a lump in your testicle, then you should seek medical attention to evaluate the cause of your condition.