The colder temperatures found in the testicles are usually not a problem for sperm. However, it is important that the testicles are not exposed to extreme cold. The temperature of the sperm needs to be kept between about 4 and 20 degrees Celsius for the best results. It is recommended that ice packs should not be used on the testicles, as this could lead to ice burns and frostbite.
The reason that sperm are cold is that they prefer to be cooler than body temperature. This is why they are protected in the scrotum, where it can be several degrees cooler than the rest of the body. It is also why men in colder climates tend to produce more sperm than those in warmer areas.
If the sperm are too cold, it can cause problems such as "blue balls", which is actually a medical condition called epididymal hypertension (EH). This occurs when pressure builds up in the epididymis, the coiled tube that joins the site of sperm production with the vas deferens and then into the outside world. This causes the blood vessels in this area to become constricted, giving the sperm a blueish-grey tinge.
While this is not a major concern, it can be a sign of other health conditions that should be investigated by an expert sex doctor. The usual colour of semen is whitish or ivory like, but other colours can indicate that something may be wrong with the reproductive organs or the seminal vesicles that produce and carry it. For example, yellow coloured semen can indicate that the prostate gland is inflamed and needs attention.