Here we see the colored belts as used in Jiu-Jitsu. You start as a white belt and work your way up to the black belt. Why is it that most people fail to reach the black belt level? Firstly let me say, if everyone would gain his black belt, it would be quite crowded at that end. And second, martial arts are nothing different than any other field. Many play a few tunes on a guitar, but there are fewer Jimmy Hendrix.
The reason people drop out are various and they are to begin with. You can train at Ichinen Bridport the style of traditional Japanese Jiu-Jitsu with Master Horst D. Lindenau who is a martial arts teacher for 30 years.
A different system of grading and colored belt can be found in BlackCombat, which is a modern self defence style. At BlackCombat ( https://www.blackcombat.org/ ) you start at the white belt level, followed by the brown and after that comes the black belt. Come along and try out which style suits you best.
You can study both in a group or as a private student.