The ejection of semen, (properly called "the ejaculation of semen") can be a combination of watery-like fluid and a white, yellow, or even dark yellow gel.
How does a man know if he should see a doctor? If you notice any drastic changes in your semen (or if you are in pain), you should consider asking a professional doctor or nurse about unusual symptoms. Red or pink colors are usually an indication of blood in the semen. Even though this would be very scary to see unexpectedly, it is usually not a serious problem. The cause is usually just a broken blood vessel that can heal by itself. However, it is best to consult a doctor if the semen is red or pinkish so that any possible helth problems can be investigated (especially if it continues for more than a few days).
About one-third of the contents of semen is actually produced in the prostate gland. Almost all of the rest of the semen is produced in the two seminal vesicles. The testicles only contribute a very tiny portion of the ejaculation contents. However, the sperm are produced in the testicles, and that's the most important ingredient in the ejaculation.