If you see a white cockroach, you may think you have stumbled across something extremely rare, perhaps an albino roach. In actuality, all roaches are white for short periods of time throughout their lives due to the molting process.
Roaches are arthropods, meaning they are invertebrates with no bones. In order for them to effectively move their legs, wings, etc., they need hard outer bodies called exoskeletons. These exoskeletons do not change size once they have hardened, so roaches and many other creatures go through a series of molts that allows them to grow. Cockroaches typically go through 7-8 molts before reaching adulthood. When it is time to molt, a cockroach will split its exoskeleton down the back and crawl out of it. The new exoskeleton underneath is necessary to keep the body together and to maintain moisture, but it is still soft and pliable. It also has no pigmentation, meaning that the roach is white. The exoskeleton will harden and darken within minutes or hours, depending on the species.
During molting, cockroaches usually stay hidden deep within their harborages. Shed skins can often be found behind appliances, inside cabinets, under sinks, and in other common roach dwellings. When they are molting, cockroaches are extremely susceptible to predators because their soft bodies make it difficult to run and move. This means that they will rarely venture into the open. So, although white roaches are not nearly as rare as you might have thought, seeing one is a fairly unusual experience.