It all started when Jigoro Kano taught judo.
Prior to the 20th century, martial arts as a practice was taught only to those in need of fighting skills. They're not looking for trophies, medals or public recognition; they're looking for defensive/offensive skills. The only "ranking" they may receive is a menken (license) from their instructor recognizing their proficiency in various aspects of training, and the final license will be a menken kaiden (full transfer license). In addition to being a skilled martial arts master, Jigoro Kano, the founder of Judo, is also a famous educator in Japan. As he began introducing judo into the Japanese education system with more students, he needed to differentiate between ability levels to keep many students on track learning throughout the system.
He borrowed the ranking system from Go, divided his students into two groups, Mudansha (unranked) and Yudansha (ranked), and developed a banding system to distinguish them.
The belt colors for the six levels (Kyu) of Peony Yarn are white, beginner, brown, and intermediate. Black is the color reserved for the tenth level (degree) of Yu Dansha students.
Later, starting from Western countries, in order to make it easier to identify, the color of the belt also joined the ranks of Kyu. Anything they want. Not a formal uniform. It's best to wear what you're actually going to wear when training for a fight. Everyday clothes. Farmers would train in work clothes. The same goes for workers, fishermen and shopkeepers. Preferably loose and made of strong fibers. And clean, out of personal preference and consideration for companions. If their outfit includes an obi (a loose obi that ties around the waist and holds a kimono, jacket, or vest in place), then the obi should be clean, well-maintained, and "not special."
In the early days, when there were no colored belts, and when men were simply training skills, there was no need for formal ranks and badges. In a small training group, each member becomes acutely aware of his or her relative position on the skill hierarchy and takes personal responsibility for his or her own growth and improvement. Years ago, one of my teachers was asked: "If we invented a new martial art today, with no ancestors' idea of ??what we should look like... what would we wear?"
" Probably jeans, a t-shirt, and running shoes. We'll be in someone's backyard. And there'll be beer."
Note: There's a myth in martial arts that students never wash their belts. A person's experience and ability can be assessed by the amount of dirt accumulated over years of training. Fake!
Wearing dirty, smelly clothes to train is disrespectful to teachers and classmates, and it is also a poor evaluation of your personal qualities. Responsible students always respect their comrades by practicing proper hygiene.