@Beau Skunky
 
why not? sweetie belle makes anyone smile with or without magic.

I think this foreshadows a future episode where Sweetie learns to put her horn to good use. But I said the same thing when her horn sparked last year, and there was no follow-up in that season.

@WinterCutieMark  
Except for the fact that Applejack, who was standing right by Spike, didn’t see it.
 
We also have an alternate explanation, which is that Spike noticed when Twilight started to be able to move her plow easily, which Applejack also noticed. Given that Spike was familiar with Twilight’s phsycial and magical capabilities, that seems like plenty of explanation.
 
There are also a bunch of other times when people fail to notice the auras, and most importantly, Twilight describes motion without a visible source as unicorn magic. MMMystery on the Ponyville Express, if you’re curious.
 
While some spells are visible, the colored auras aren’t, and even the visible spells don’t seem to be color coded.

@Sjogre  
I’m not so sure about that, Spike was able to tell that Twilight was using magic back in Winter Wrap Up, and the easiest explanation for that would be that the magic aura IS visible.

@WinterCutieMark  
And Shining Armor and Twilight’s mother share Twilight’s magic color.
 
Really, the magic aura colors are just to aid the audience and don’t exist in the show, so the only rule is whatever will make it easy to identify the user.

@Applebloom121  
From what I can tell, more often than not their magic colour can be found in their cutie mark, but in the end there seems to be no rule that works for all characters. Trixie for example has an eye colour based magic aura, while Princess Celestia has a cutie mark based one and Rarity has one that could be based on either one.

[Hovering intensifies]

@Frostwolf123
 
magic is the color of there eyes

Sweetie Belle’s magic brings a very big smile!

@Frostwolf123  
Nah, that’s the effect from the black vines.

Interesting observation. In the Babs episode we saw the Sweetie Belle’s magic is green. I’m pretty sure that was still Rarity’s magic doing that. Way to almost break your sisters neck Rares.

THE POWER IS CONSUMING HER

“Does not compute.”

@Fortune  
Nah, just siblings beign siblings. Nothing unusual here.

I wonder if it’s like this for every unicorn when they first start developing magic abilities.

AN ERROR HAS OCCURRED

I don’t really know what it says that Rarity’s first instinct for things going haywire is to blame Sweetie Belle.
 
Pattern recognition?