I like the idea -- which I have read a few times over the years from different sources, so I don't know where it originated -- that in D&D there are only really 2 species of dragon: chromatic and metallic. A chromatic dragon goes through all the colors, molting both as it ages and when it moves environments. Sometimes a dragon just decides to stay someplace and thus you get an Old White, but much more commonly a dragon hunts out its territory and has to move on. Therefore it is much more likely that any of your Ancient dragons are going to be Red or Blue or Silver or Gold.