Belt system in different countries vary. In the US, it is like this: http://220.127.116.11/mt/Belts.asp
But in Taiwan, take a look, you can see a little different: http://zh.wikipedia.org/wiki/%E8%B7%86% ... 7%E8%A1%A8
But the meanings of the colors are the same. And I believe that over black belt the ranking is the same in all countries, because level 4 and up must be certified by World Taekwondo Federation (WTF).
There are similar belt systems in Karate and Judo too.
I am not so sure how "universal" the belt system was until modern times. As far as I know (and I of course could be wrong here...) its a pretty modern invention, so I would not put too much stock on there being any actual significance to the color.
From what I have heard by most martial artists, back in the old days, you started off with a white belt, to keep your suit closed. After training, sweating, bleeding and being all nasty for years, the belt turned black.
Many chinese martial arts do not use a belt system at all.
And I know of martial arts that do use a belt system, but it goes white->green->green->green->green->->green->green->green->BLACK
Soyeah, I think the whole belt thing is pretty new...its good for marketing. I seriously
doubt the medieval samurai were given different colored belts as they studied (again, could be wrong....but from EVRERYTHING I have heard, read and experienced, the belts are just for us soft modern folk to give us instant gratification)
BTW, I responded to your "significance of 5 & 3" question last night, since I just discovered it.