@Cirrus Light  

don’t be silly. a moustache is made of moustache.
 

@redweasel  
The idea is if you can grow a mustache, can’t you grow hair, since a mustache is made of hair?
 
Maybe it only works since it’s not actually pony hair or something, I dunno. Maybe like all their other attempts at fixing it, it just wouldn’t go right for longer than 10 seconds.
 
Honestly she just needed a good mane for a photo shoot, though, she didn’t have to travel with it as long as it’s okay if the photographer knew about the hair condition, they could just have Rainbow make that cloud mane at the photo shoot. But probably she didn’t want Photo Finish et al. to know about her condition :q
 
Anyways, at least the whole “no hair magic” thing is consistent with the fact that there’s no cure for Cranky Doodle’s baldness, though.

I think this is actually a really interesting callback to some little known lore way back in G1. (little known, because the episode was so bad that nobody could stand to watch it, not even little girls.)
 

 
pony hair was used to weave a magic cloak capable of glassing the entire planet. it’s extremely magical. it also grows back super quickly and styles itself.
 

 
never mind how ponies seem capable of hair magic.
 

 

 

 
so something capable of disrupting that is not a thing I would want to casually meddle with, much less the hair itself.
 
plus I fail to see how it would solve anything to give rarity a moustache.
 

@Cornerverse  
Because nobody understands that’s Rarity’s magic if you don’t use the established color of Rarity’s magic. Otherwise we just wonder if you forgot to or did some stylistic thing and didn’t draw Twilight’s horn glowing.
 
Looking up ‘‘Rarity, magic, screencap’’ , there are 547 images, and in maybe 80% of them Rarity is using magic, and it’s blue. Excepting the time her magic was tainted in Inspiration Manifestation and was green, I only found 3 exceptions to her magic being blue, either because Rarity was Radiance in Spike’s comic book, or a screencap from “Look Before You Sleep”, when the show artists were still deciding how magic should look (faint glow with sparkles as opposed to the more solid glow we got for the rest of the show).

that’s trixie’s magic! rarity’s been framed!
 
either that or trixie is rarity’s secret assassin

I realized this while I was trying to sleep. I pulled myself out of bed to see if anyone realized this first.
 
… And someone did, thank you.

@Cornerverse  
you know besides the whole magic color thing, whenever a unicorn uses magic their horn glows

@Background Pony #10BD  
But Twilight’s magic is Magenta? Like, very obviously Magenta to the point that Blue-Violet would be just as confusing?  
Purple is a color associated with both ponies, though admittedly Twi has more. That said, she only has Lavender or her straight up Purple mane streak and eyes, while Rarity’s Purple mane is a Blue-Violet like the magic I used. And were it Twilight holding the knife, her horn would be lit up as well.

I think it’s because it’s easier to grow facial hair where it doesn’t belong than to restore a mane where it does belong. Headcanon it has something to do with manes being where excess magic is stored, which while I’m on that headcanon, maybe to make an alicorn mane flow, it works like the Strength of a Hundred Seal from Naruto, but larger amounts of time for the stored magic.

@Cornerverse  
Put bluntly? Because we’ve come to associate purple with Twilight. Until I read your comment, I thought that it was implying that Twilight was about to murder Spike so that Rarity would never find out.

@Background Pony #82DF  
Tbh, the magic auras shifted a lot back in the day, and this was the one I liked best. Besides, if the show can’t get it’s own continuity right, why shouldn’t I use the color that I think works best?

@Cornerverse  
Wrong aura, it’s blue

@Alkie  
Back with the moustache, Twi made it dissapear. And they also specifically called it ‘growing magic’ instead of an Illusion.  
@Forest2TS  
(If you’re asking about the knife it’s Rares off screen)

… well, Considering how fast Spike’s mustache disappeared, the spell she used was either very temporary or illusuory.
 
Thus, the spell wouldn’t work in the long run

Is Twilight’s horn glowing?