From the gamist point of view, there are plenty reasons to divide magic into colors in an RPG
- increased replayability
- increased difference between various PC casters
- increased customization and sense of identity. Say that novice wizards start with one color, expert can add a second and masters can add a third That makes for organic character customization which comes up through gameplay, not at character creation. Much more if you pair it with all or some spells being "found" in-game.
- different types of wizards build world. Factions, tensions, zones on the map that belong to ones or others. The whole Kanto is built over making zones for each pokemon type, but the same can be said about Ravnica.
- the opportunity to create different legendary spells or magic items tailored to specific types, which can be quested for by their respective PCs. This type of "item hunting" is one of the best things you can have when you play a sandbox.
And, lets face it, I want to create something inspired by Pokemon Magic since ages. It's one of the best examples of good gameplay-oriented worldbuilding in history, and there is a lot to steal from it.
After thinking a lot about this, I think that the correct number of schools for my project should be around four or five in the book, with around 10 spells each (lets say: 6 basic, 2 expert and 2 legendary). But instead of closing them in a wheel (as MTG does) keep it open so one could create custom schools or spells around any concept one should want (chronomancy, technomancy, etc. Sense of taste not included) while still being compatible with the existing lists. As I was doing my research, I found out that the uneven GLOG does "in spirit" much of what I am striving to do, but of course, I am going to do it my way anyways.
So, first of all, lets see some examples of magic schools portrayed elsewhere.
You got eight of them in post-3e D&D: abjuration, alteration, conjuration, divination, enchantment, illusion, invocation, and necromancy. This particular case is interesting because types are defined mostly around their "role" in game: alteration and illusion are suited for adventuring tricks, conjuration and invocation are more suited for combat, with necromancy having a very defined use. To put it in a way, wizards are much more "pigeon holed" in what they can or cannot do.
Pokemon, on the other hand, is divided by theme (lightning, plant, water, etc) but the role of every theme is mostly the same: beat the other guy down.
Magic the Gathering's five colors fits nicely between both examples: Colors have a solid definition in theme, and while all of them can take you to "zero life" they play wildly differently. And while they have limitations and specializations, they feel like they are built around a theme in-game and not around utility in a design room as 3e D&D does.
Ideally I want to conceive five wizard schools and try not to rip off MTG straight in the process. Five is a curious number to base something, as there are much lesser correspondences than with the number four (four directions, four elements, four seasons, four quadrants on an X/Y graph); though it is used on chinese systems (earth-metal-wood-water-fire) or japanese ones (wind-fire-water-earth-void). Also japanese use a five season calendar that divides summer into a rainy and a dry season.
So, to begin with, here is a table in which (you/I) can roll up some wizard schools. Your school spell list gets one spell for each type below, but for one that is unknown to them (roll). Two other types get one and two extra spells respectivelly, for a total of 10 spells. A school with three types of offensive spells is likely to be a very quarreling faction, while one that focus on alter the self or alchemy will look more like a sect of cultivators.
1 offensive (damage + certain status alterations)
2 healing (damage and or status alterations, including death)
3 divination (prophecy, ESP, commune-style questions, etc. Tapping into the "hidden" side of everyday things, such as travelling through mirrors or speaking with animals, also goes here)
4 summoning (other entities, forces or objects)
5 enchanting (alter things and people, also non-violent combat moves such as sleep)
6 altering the self (transformation or other power ups)
7 alchemy (preparing potions or other consumables, probably buffed in uses to compete with instant spells)
8 protection (any kind)
Now, roll two or three times for the themes of the school. These will help you give form to the spell list, and hint which forces or mythological animals power your advanced spells. Just forget for a while that these are the official pokemon types.
1. Fire
2. Water
3. Grass
4. Electric
5. Ice
6. Fighting
7. Poison
8. Ground
9. Flying
10. Psychic
11. Bug
12. Rock
13. Ghost
14. Dark
15. Dragon
16. Steel
17. Fairy
18. Normal
Suit yourself to choose what each of this words mean. Fairy in the pokemon game is used mostly as mind-alteration (which make more sense on the Psychic type IMHO) but it can be interpreted in a more open way and make it about fate, bending space and time and other works of elves and demigods. It feels natural to expand Ghost into necromantic/exorcist territory, while type: Normal is probably best used as representing animals and other beasts (druidic style)
Now roll twice for your thematic colors
1. Red
2. Blue
3. Yellow
4. Orange
5. Purple
6. Brown
7. Black
8. Green
9. Pink
10. Indigo
11. White
12. Emerald
13. Lavender
14. Turquoise
15. Gold
16. Silver
17. Bronze
18. Cyan
19. Magenta
20. Go monochromatic. You can always choose this instead of any result. If this is your only result, roll again.
EDIT: link to Dont worry, I've got a Sword where the author has worked on the same topic.
EDIT 2: 8 Schools of Magic by Reckless Dweomer