Uniform requirements vary from style to style.
In the style I study, the gi is white. It is always white, or a natural fiber color, never black or any other color.
Some styles allow instructors...
It usually represents a rank. i.e. White with a stripe (either a new belt with a stripe in the middle, or sometimes just black tape at the ends) it a higher rank than just a white belt. Yellow is...
It will depend on the style of karate. In some styles, a blue belt is one of the kyu levels. In others, a blue belt is used instead of a black belt. In WKF , the blue belt (ao) and red belt (aka) are...