I Made A Thing · 10:53pm Sep 12th, 2012
FOREWARNING: This is firmly ensconced in Magic: the Gathering related stuff. If you don't care about the card game, feel free to close the tab.
Anyway, as noted above, I made something in light of the impending Return to Ravnica. I thought it may be of interest to you all.
I also made some rationalizations justifications explanations for why I put which pony where:
Fluttershy: Cares for a vast number of creatures of all shapes, sizes, and diets. Gentle, but defends others ferociously.
Rainbow Dash: Passionate, aggressive, volatile, charismatic, and fanatically loyal. Subtle as a brick. Highly goal-oriented.
Rarity: Finicky, detail-oriented, controlling, contemplative, and organized. Good intentions, but not necessarily the right priorities.
Applejack: Understands, appreciates, and uses the natural cycle. Advances own ambitions by helping the greater community. Unrelentingly stubborn.
Twilight: Passionate thinker. Consumes information ravenously across a wide gamut of topics. Researches and experiments obsessively, moving from one topic to the next as her interest shifts. Fluctuates between inflexible and unstable mindsets.
Pinkie Pie: Hedonistic, carefree, parties like there’s no tomorrow (and occasionally makes it less likely there will be.) Completely unbound and unbindable by rules or restrictions.
Celestia: Has established a utopia of peace, joy, and nopony questioning her right to rule. Adored by the unquestioning masses. Cares for and subtly guides her people.
Luna: Rarely seen, thought of more as a boogeymare than an actual poltical figure. Shapeshifter, illusionist, lurks in the shadows by definition. Subtle enough that the writers apparently forget she’s there at times.
Lyra: Obsessively fixated on a perfect biological ideal. Focused on goal, not self; keeps to the background. Unwilling and unable to stop her obsession. Believes it’s for the best, but that nopony else would understand or appreciate it. (Yeah, had to resort to fanon on this one, but FiM doesn’t exactly have a lot of singularly obsessed science ponies. Maybe Hugh Jelly.)
Screw Loose: Impulsive, instinctive, incoherent. Feral, even. Responds only to simple commands ("Crush them!", "We eat!", et al.)
You may now explain the myriad ways in which I am wrong. 
This, of course, leaves us with the question of where somepony grey fits into things.
354883 Depends. My Ditzy tries to stay out of guild politics. She tends to stay out of Ravnica in general, really, though that's more a matter of consequence than conscious choice. If she had to pick, her magical preferences would probably pigeonhole her into the Azorius Senate, much to her displeasure.
A more canonical Derpy could would probably either end up busting pavement for the Gruul or acting as a Boros shock trooper. Or she might be the best Dimir sleeper agent ever.
Maybe I'm missing something, but the rationale you gave for Celestia sounds extremely White. I'm not seeing a trace of Black, other than the simple fact that she's the one in power. But then, I know next to nothing about Ravnica, so maybe it's some aspect of that specific guild or something.
355248 The Orzhov Syndicate is what happens when you combine organized crime and organized religion. White-black in general is the conflict between morality and amorality, idealism and cynicism, group and individual welfare, and so forth. It's by far the most political of color pairs, having to constantly compromise and cooperate while never being able to completely trust its fellows. When working together, the two work to advance a specific group, white thinking of what's best for the members of the group, black advancing the group as an extention of itself. White-black wants a blend of peace and omnipotence; it wants to be the world's peacekeeper. Rules and social structures are the primary tools to help further this goal.
Celestia is certainly closer to the white end of the spectrum, but as the god-empress of a quasi-utopian society where the most dangerous weapon is pastry, she's living the white-black dream.
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In short, the Magic mafia which is ran by ghosts.
Does necro even apply to comments like that? Anyway, since you linked to it...
I'm not too much into MtG but White/Black seems like a fairly natural combination. You need Power to Protect the weak and enforce your Order and your Morality and your Peace. The guardians of the ruling Order are by nature and necessity Powerful. To try and impose your Order upon the world is a great Ambition, it's Selfish and even Parasitic in its own way and one must seek Omnipotence in order to accomplish it. Paranoia and Pragmatism are necessary for victory, because the rest of the world is not as Moral and honorable as you. Paranoia is also justified because any of your allies may develop their own Order to enforce upon the world instead of yours. Basically every trait from the Color Pie matches up.
Wouldn't rarity fit into simic the best since she has a need for designing things and putting what she has made to use
In my idea I would switch Twillight and Rarity but the rest is just perfect