The only colour that I know that means something is he dragon in the middle, which is always yellow. Yellow is emperor's color. There are 9 dragons because 9 is the largest of all digits. And 5 is in the middle of all digits (there is no zeros here), so they make the fifth dragon, the dragon in the middle, the emperial yellow dragon. The one in the middle is also one that faces forward, while the ones on the side, which are always paired fron the center, will either face towards or face away from the dragon in the middle. Although, as far as I can remember, I have never seen 9 colored dragons on a single wall. Becaus they are always yellow in the middle and 4 pairs of dragon on the side. So 5 different colors on one wall maximum.
Oh yeah there are a few of these 9 dragons wall n China. Yes the two of three most famous such walls ar in Beijing. The third one is in Datong. And there are a few more of them by they are not so well known.