Although I have never read the Lord of the Rings more than once in my life, I do remember that there is a suggestion of an order of wizards whose status is determined by color. As I recall, three colors are explicitly mentioned: white, grey and brown. Thus, in a world where there is a strong guild-structure, it makes sense that Averoigne would pay homage to Tolkien by differentiating different types of arcane spell casters by color:
White Magic-Users: So-called because they practice white magic — arcane cleric spells. They must be Lawful* and they cannot cast any spells above 3rd level.The Prime Requisite in all cases is Intelligence which also determines the chance of knowing a particular spell as well as the minimum and maximum number of spells known per spell level (as per Holmes). Practitioners of white magic use the cleric spell progression and XP table and may use armor, shields and blunt weapons but cannot Turn Undead. If they are Lawful and have a Wisdom of at least 9 they can choose to become full-fledged clerics at any time after 3rd level (they gain access to all spells through faith instead of study and can Turn undead).
Black Magic-Users: So-called because they practice black magic — arcane magic-user spells. They are usually Chaotic and exist outside of the guild-structure and society itself. As a result, they are considered dangerous criminals to be hunted down and (usually) killed.
Grey Magic-Users: So-called because they practice black magic in context of the Church — a mixture of black and white. They can be Lawful or Neutral and must belong to the Wizard’s Guild.
Brown Magic-Users: So-called because they are seen as close to the earth — operating mostly in villages, towns or as hermits. They use arcane cleric spells and have no formal guild. Since they practice white magic, they are generally left alone (which opens the possibility that individual brown magic-users may have a spell or two from the druid spell list). They tend to be neutral and can gain a maximum of 6th level in experience.
Red Magic-Users: So-called because the casting of reverse cleric spells normally involves some kind of blood sacrifice. These are anti-clerics. They are Chaotic and exist outside of the guild-structure and society itself. As a result, they are considered dangerous criminals to be hunted down and (usually) killed.
*Assuming one is using the three-tier alignment system.
There is also a third type of arcane magic — sword magic. It interferes with both white and black magic. The latter is the most susceptible and therefore practitioners cannot wear any armor and can only use simple weapons. White magic is less susceptible, but practitioners cannot use bladed or piercing weapons.
Technically, this means that clerics could use any type of weapon they choose (since they cast divine magic, not arcane magic); however, by tradition they limit themselves to blunt weapons to prevent any use of sword magic lest they misplace their faith in God with a faith in sword magic. Thus, any cleric that uses a magic sword cannot cast any spells, because they lack the faith to do so (or believe that they are called to use sword magic instead of divine magic). This opens up the possibility of a paladin-like cleric who can Turn undead, use magic swords (like a holy avenger) but can’t cast spells.
I don’t know how attractive any of these options are to players, but they open up a tremendous amount of opportunities for cool NPCs, cool villains and cool patrons for yours truly.