One: Must be wearing purple for your flaw.
Two: If you want to create purple dye without Vis you can do so! Use Art and Acedeme to create a regent that turns cloth purple. You can even create a rego spell that creates the reagent in a single casting although it would have a prohibitively high finesse requirement.
Warped magic: Clothes start turning purple. Stronger spells affect more and more clothes (on others near you) and make the clothes a deeper purple.
Necessary Condition: You must be wearing Purple.
Deleterious Circumstances: when not wearing purple
Side Effect (Virtue): The lovely smell of Lavender accompnanies your magic
Or bring it as a Lab Text with build points for someone else to learn? Ophelia will likely have an Int+Finesse total in the 8-10 range. Or we ask jebrick to have it in the list of lab texts in the covenant?
Necessary Condition: must be wearing purple is a very dangerous flaw. Purple, as it was pointed out to me when discussing the Weak Magic Resistance : caster wears purple flaw, is a very rare color in Mythic Europe. If I lose my purple clothes, I will be stranded without magic for a long time... including any magic I may create to make purple. I'd have to basically find the closest bishop or prince and convince him (without magic) to lend me his clothes !
The description of Necessary Condition and the canon examples are all of activities/circumstances that are much easier to fulfill. Clapping hangs, spinning, singing, touching stone (it's everywhere !), Even Necessary Condition: casting tools, the implicit flaw of the Verditius, isn't that bad: casting tools can be created in an hour.
Can't have it both ways: if it's too difficult a condition to fulfill for Weak Magic Resistance, then it's overkill for a Necessary Condition. And Weak Magic Resistance made so much more sense: faced with such a symbol of Imperial authority, Diotima's defenses instinctively go down... whereas Necessary Condition : wearing purple ? Diotima doesn't harbor the delusion she should claim the throne herself (the Order would have something to say about it !)
Now if it weak magic resistance when faced with faced with a symbol of Imperial authority I could see that. People clothed in purple or carrying/wearing other symbols of authority
Added exposure experience for Peripha (46 xp total, he's learned to read, be charming, be graceful with his powers, filch things, and play the cymbals; whether that's an improvement depends on who you ask).
Added Diotima's aging rolls (as in, no apparent aging).
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Added the True Friend virtue to both Diotima and Peripha, as a result of the bond.
Changed Diotima's Weak Magic Resistance to be to "casters wears a symbol of authority", which should be easy enough to contrive to qualify (crowns, scepters, the right clothing, bishop's crooks, the right blazon...) The symbol doesn't have to be real, and the caster doesn't have to have any right to wear it; all that's needed is a decent approximation.
Idle question that doesn't really matter: Does the 'grace' specialization of Finesse applies to the Int+Finesse roll to make purple dye ? Inasmuch as the roll is to make beautiful dye, after all, and Diotima is a Jerbiton even if her strength lies not in her artistic talents. Not that it really matters, I'd just like to know.
It is a good weakness. Not your everyday occurence, but common enough to be a hindrance. And quite easy to get for enemies if they learn about it! I like it 
I would accept your specialization for Finesse to apply, but then I am not a member of your saga, only a lurker, so you'd better wait for the other people here to comment on this one
Xavi
You need to spend Xp on these and not do lab rolls.
I also did not see where you taught your familiar magic theory. I might have missed it. If you do not teach it, they do not have it.
Now you tell me about the spells ! and you missed some by the way... 
goes back to the drawing board
But if you are so set upon using the rules for advancement, why did you limit the amount of vis we can use in character generation so much ? It's right there on p.32:
Among other things, it makes it difficult to get a decent LR at age 35 and almost impossible to get anything else done early 
You took so long to answer to my query about teaching the familiar, I went ahead and gave him Arcane Lore instead (see backstory), which gave him Magic Theory. I'll rewrite and teach him instead...
All right, I rewrote Diotima and Peripha (see the first two posts).
Cuts and downgrades
- downgraded my Mentem from [spoiler]35+3 to 29+3[/spoiler]
- cut out one spell (Loss of a Moment's Memory) and one Bond Enchantment (Seeing and hearing through the familiar)
- downgraded the familiar Bond level to 45
- did my own LR at age 35, it's only +6
- Peripha doesn't get Arcane Lore or a built in Magic Theory
- cut out some spell Mastery Abilities
Upgrades
- Brought forward the Bond to age 35, this increases the amount of exposure Peripha gets from me
- taught him Magic Theory one season, for the principle of the thing, before letting him pick up the rest from exposure
- spent some of the freed vis on increasing the talisman capacity (adding one attunement, +5 control people)
- Learned level 2 Mastery Ability for my one offensive spell, Gaze of the Cockatrix (Magic Resistance, just try putting me to sleep with ReMe now, and Fast Casting)
Various Changes
- Add to reorder the learning a bit to get my max CrCo lab total at age 35
- all spells are now bought with xp
- added two music instruments to my list of belongings
- added one casting of Color of Imperial Pride, my purple dye creating ritual, and added purple dye and purple ink to my list of belongings
- recomputed my aging rolls, keeping the same dice rolls but changing the modifiers to take into account the new values
Vis isn't limited, per se, it just needs to be accumulated at the rate of 2 pawns per year, or take a season of no XP and earn it, as outlined. So, because she started at age 25, she could accumulate 20 pawns of vis for her LR,without even doing a season of lab work. 20 pawns buys an LR 11 (7+(10* 1/magnitude up to ten)+(2* 2/magnitude up to 20). Take another season off, and you can push it to LR 13. This is really imporant with a Verditius in the mix, since they could create tons of things with 5x Magic Theory in vis available to them.
I suggest modifying your Call to Slumber variant even further. Make it D:Diameter and you can keep them asleep for a couple of minutes. Putting to sleep for a moment is good, but if there's combat around, they'll wake up soon after. If you can keep them sleep for the duration of combat, that's pretty powerful.
Might want to verify how you perceive fast casting working, it may not preempt another party's action. Fast cast spells should be defensive in nature and respond to whatever event would take place (not the events that preceded it) This was a recurring issue in Bibracte, and I think jebrick viewed it as I do. You can Wizard Leap away from an attack that would otherwise hit you, you can counter spell. And you can do Mutually Assured Destruction: use an attack spell, which will go off, but won't prevent the opponent's attack from happening.
Say Ophelia is messing around with her grogs and runs into Marched Magus Alpha and some of his henchmen. Everyone girds themselves for battle, and it is joined. Ophelia is really fast, and she goes first, so she buffs two of her grogs with Blade of Virulent Flame to handle his henchmen more easily. Alpha issues orders and then makes himself invisible. The grogs melee with each other. Now, say, Ophelia had Invisilble X of Vilano mastered for fast casting, she couldn't use it to get a parting shot at Alpha before he goes invisible. She could unravel his spell if she knew Unraveling the Fabric of Imaginem, but she doesn't. Fast cast options should be defensive in nature.
For your spell, a combatant might come swinging at you, having taken out your shield grog. You fast cast your sleep spell, he'll go to sleep, but it will happen after he's sliced you but good.
I will put something up on fast cast as it has been hashed out. But basically, JL is correct in my views. It is not a preemptive strike. It is in reaction to another action. You cannot bypass initiative with it.
Good points on Call to Slumber and fast casting. And if I drop the second level of Mastery I can easily afford that Diameter upgrade. On the other hand this is my only combat spell, so there's really no other candidate for fast casting.
Regarding vis, my first draft had me starting my Talisman early, and enchanting the effects as soon as I could. It also had me binding my familiar relatively early. And of course there's the LR. Well, I ended up using a few more pawns of vis than was available, easily fixed since I had plenty of unused capacity in my talisman, but it also had me using them years before I had them to use. In my latest draft I manage to bind my familiar early at the cost of a weak LR and a lower Bond, and I'm back to almost my full initial talisman. So it's more the lack of flexibility that hurts me rather than the low limit.
One of the things that Diotima could do is Spontaneously cast ReMe effects with her effective ReMe CS of 42, halved for spontaneously casting a spell, she can kick of 20th level effects pretty easily, but it will also appear that she doesn't know these very well, because she fatigues herself. Given her Mentem score, Magic Resistance probably isn't useful, but multiple casting is.
Eventually over time, people she associates with and observe her working Mentem magics might pick up on that she is working a spontaneous spell, because she does things differently all the time for the same basic effect. Now, you do take a risk using spontaneous magic, and it's one that Diotima should be considering learning all the Mentem spells she's likely to cast. In short, there's some compelling stuff for her to wrestle with, learn the spells and drop her pretense, whether immediately or gradually. But, despite her dark secret, there might be some who know, Dark Secret might not be the best way to represent this, but I'm not sure there's any other better way.
No one shall mess with Diotima's mind ! 57 Magic Resistance vs Mentem is nice, but 114 is better ! (She's slightly paranoiac that way, probably as a result of having it done to others so often...)
You are right, if she absolutely has to she'll whip out the spontaneous TeMe effects, particularly if there is no Hermetic mage to see her do so.
Fact is, Dark Secret is a Story Flaw. So by taking it I'm telling the ST to include it in a story eventually. That said, the Dark Secret isn't that she has a high Mentem score, it's that she's being breaking the Code by interfering with mundanes, specifically the Latin rulers of Constantinople. Diotima is designed as a social magus, I expect to take her on any diplomatic mission, not in the missions where combat seems likely. That's what my companion will be for. It could be a very long time before she has to spontaneously cast the same effect in front of the same magus twice...
That's true, regarding the length of time between Mentem effects. And, I did get your Dark Secret on, I just remembered thinking that's not much of a Dark Secret and applied it to your Mentem score. Too many secrets to keep straight. 
The reason why I don't think your Dark Secret is much of one, is that magi interfere with mundanes all of the time. It's only a crime if you get caught. Some of your spells may not work properly, or are perhaps a bit too showy for doing it in a city. Quiet and still masteries would be imperative, since she doesn't have Deft Mentem. So, if she works a Mentem spell on someone in Constantinople, she'll be noticed babbling in Latin and waving her arms about to get the spell off. If she goes no voice and no gestures to not get noticed, and she's in a city with an average divine aura of 3, she's -24 to her CT. Given that, I'm not sure exactly how capable she has been, or how unnoticed she has been.
You did notice that Diotima has a number of Arcane Range Mentem spells ? Including one to read someone's mind, one to plant suggestions, and one to kill. (also a shapeshifting familiar to collect arcane connections in the first place and so some more spying). Diotima did go to Constantinople once, years ago, for a brief visit, but she was the model of a perfect magus then. She waited until she was out of the Theban Tribunal to start her campaign. Since then she has been deliberately undermining an Emperor to help a foreign invader conquer a country, so she has gone quite a bit beyond what other magus typically do.
As for getting caught, since this is a Story Flaw, it will happen, no matter how unlikely, and despite the considerable protections she has erected against it (using Shroud Magic, residing in a different Tribunal, hiding her knowledge of Mentem and choosing her victim with care). She might be able to bribe her way out of it, or PeMe her way out of it, or argue successfully that it doesn't count as a crime, but sooner or later she will get a story out of it - which is all a story flaw is supposed to do.
I'd say that it's possible to get out of it, not fait accompli that she will get out of it. Diedne Magic, which gives the Dark Story Flaw doesn't suggest that the character will eventually get out of their Dark Secret. If we could presume that one always gets out of it, then...
Ophelia took Favors (Tremere) with the expectation that she gets jobs that are dangerous enough that Tremere would rather not waste their brothers and sisters of Tremere on. Doesn't mean she's going to make it out of all of them alive.
I didn't say she will get out of it with a story, I said she will get a story out of it
Pardon my skimming. It bites me in the a$$ sometimes. Maybe it's a personality flaw? 