Jun Akiyama wrote:
And to hopefully head off the inevitable "black belts came from a white belt gone dirty" urban legend, I'll continue to quote from the FAQ: "An often heard story holds that the colours are explained as follows: a trainee's belts, which, traditionally, were never washed, became progressively dirties with time (starting out white, becoming yellow with sweat, green with grass stains, and so on), finally changing to black over the years. This explanation, alas, is almost certainly fanciful."
I'm into my second decade as a mudansha, and I can say that this is not entirely an urban legend. I have one belt that has lasted me throughout the years. It does get washed regularly, but as the outer covering has frayed and worn away, the inside, which is black, has become increasingly visible. It really is kind of cool, and I feel it does represent the time I have put into the art, as I gradually creep toward shodan.
However, I might feel a bit different about the whole thing if the stuffing was, say, hot pink...