Random Roll: PHB, p. 32.

 A close reading of the Advanced Dungeons & Dragons rulebooks often reveals little details that are easy to overlook. In most cases, these details are rules-related, but one occasionally finds details that pertain to the implied setting of the game. I came across one of these just recently while re-reading the description of the monk class found in the Players Handbook.

There can be only a limited number of monks above 7th level (Superior Master). There are three 8th level (Master of Dragons) and but one of each higher level. When a player character monk gain sufficient experience points to qualify him or her for 8th level, the commensurate abilities attained are only temporary.

The above is, I think, well known and has been an aspect of the class since its first appearance in the pages of OD&D's Supplement II. Like the druid, the monk is a class that advances to higher levels only through the defeat of the current holder of that level in a trial by combat. I know that, even back in the early days, some players and referees disliked this aspect of the class, both because of its seeming unfairness – why don't other classes have to do this? – and because it introduced an additional layer of complexity to leveling up. For myself, I liked it precisely because it was unique; it gave the monk a bit of flavor to distinguish it further from other classes.

The next sentence of this section of the PHB also contains a bit of flavor, but one that I must have somehow overlooked, because I honestly cannot recall ever reading it before.

The monk must find and defeat in single combat, hand-to-hand, without weapons or magic items, one of the 8th level monks – the White, the Green, or the Red. 

For a moment, the colors baffled me. I quickly realized that they were connected to the fact that the title of 8th-level monks is "Master of Dragons," of which there are only three. Thus, it would seem that these monks consist of the Master of White Dragons, the Master of Green Dragons, and the Master of Red Dragons. How had I never seen this detail before? It's baffling to me and yet I have no recollection of ever having seen it, let alone making any use of it in all the years I played AD&D.

The detail makes a certain amount of sense, since, unlike levels above 8th, there are three 8th-level monks, so there ought to be some way to distinguish them. Of course, I soon find myself wondering: Why only three? Why not one for each color of dragon? Is there some special significance to the three dragon colors chosen? Why are they only evil dragons? Thinking about and potentially answering these questions are the stuff from which a fantasy setting is made. I have no idea if Gary Gygax intended there to be a logic behind the three Masters of Dragons or not, but I enjoy puzzling out matters like this regardless. I doubt I'm alone in this regard.