Ever since his first appearance in the animated Star Wars: The Clone Wars movie, Captain Rex has stood apart from other clones with his blond buzz cut. Created by the cloners of Kamino, the Republic's clones are created from the genetic template of Jango Fett. Like Jango, many of the clones keep their hair black. There have been a few other clones with varying hairstyles, such as Commander Gree with his reddish-brown double mohawk, and, of course, Captain Rex. Most viewers assume clones like Rex chose to dye their hair, giving them a sense of individuality, but that can't possibly be the case.
The key problem is that Rex was an active soldier, often active on campaigns in the Outer Rim. The Clone Wars timeline lasted three years, and there's absolutely no way Rex was ducking off-screen every few days to make sure his hair stayed blond. There are campaigns where he was on the ground for too long to keep it short, meaning viewers would have been able to see changes in his hair color. However, there is a better explanation for his hair.
Rex Genetically Altered His Hair Color
In Star Wars: The Clone Wars episode 14, Obi-Wan Kenobi goes undercover as "Rako Hardeen" to stop a plot to kill Chancellor Palpatine. To achieve a perfect likeness of Hardeen for the operation, Obi-Wan has his face and hair genetically modified using a facial transformation program. This makes his face look just like Hardeen's - and alters his hair color to gray. Rex likely chose to go through a similar process, explaining why Rex's original black hair never grows out - even during long and important Clone Wars battles like the Umbaran arc.
In a sea of thousands of clones who all look identical, it makes sense why Rex felt the need to make the change to his appearance. Like many of the other clones who opted for facial tattoos or interesting hairstyles, Rex sets himself apart with his blond buzz cut, making him instantly recognizable to other characters and the audience. Granted, it's interesting to speculate about how Rex afforded this; it's possible senior figures in the clone army were allowed access to this technology for their own growth.
Despite the technology for genetic manipulation existing in the Star Wars universe, the movie and tv shows have barely explored what it can do. The Force has always been able to influence genetics, with Grogu’s blood being used for a nefarious experiment in The Mandalorian, or Palpatine potentially influencing Shmi to conceive Anakin before Star Wars: Episode I - The Phantom Menace. But there seems to be a whole world of genetic science outside the Force. The Jedi used this science to deceive the Separatists, while the clones utilized it for cosmetic purposes.
Rex’s Hair Could Be A Genetic Defect
While it’s probable that Rex genetically modified his hair, there’s also a small possibility that it is actually a genetic defect caused by the cloning process. Towards the end of the Clone Wars, the Kaminoans began to run out of Jango Fett’s original source template, causing them to stretch it in order to keep producing clones. While this caused some problems in more recent clones, it doesn’t explain Rex’s hair given that he was one of the first generation clones. That being said, there have been unique mutations with earlier clones (such as 99). Nevertheless, the most likely answer is that Rex genetically modified his own hair before viewers encountered him in Star Wars: The Clone Wars.