The colour of unicorns' magic aura is the colour of their eyes.

[Armin 168]

Alright, so I'm not the best at explaining things, so I'll use picturess to demonstrate my points. I thought of this on a bus yada yada yada.

Right, I think you get what I mean, but in case you're dumb enough still not sure what I'm saying, I'll show some examples:

(Pink is a shade of purple now. Deal with it.)Twilight Sparkle has purple eyes. 

What colour magic does she have? Pinkish purple!

Rarity has blue eyes. Guess what? Blue magic.

Now Sweetie Belle still can't cast magic, and although we saw some GREEN sparks shooting from her horn in One Bad Apple, let's pretend we didn't. What colour should her magic be?

Now let us guess what Pumpkins' magical aura should be, shall we?

 

(Note to self - Make sure you're fully awake when typing these things out to make better sense.)

Edited September 6, 2013 by SnapShot

[An Old Head 4,816]

Hey, it's true! Confirmed it for the Flim Flam brothers:

 

 

Dang, I think I might have to check every single unicorn to find a discrepancy. I'll start with Sombra...

Ah, but wait, found one! Cadance's horn glows blue, while her eye colour is purple. (Alicorns aren't disqualified from this, right?) I also noticed that Shining Armor's horn glows purple when his eye colour is blue... if you switched their eye colours around, it would match up. Might be something to do with their love for each other?

 

Edited September 6, 2013 by Flipturn

[Scootalove 10,689]

That is... actually true. The color of a unicorn's magical aura that forms around their horn when they do an action involving magic, which matches the color of their eyes. What a noticeable coincidence.

 

[Armin 168]

Hey, it's true! Confirmed it for the Flim Flam brothers:

 

 

Dang, I think I might have to check every single unicorn to find a discrepancy. I'll start with Sombra...

Ah, but wait, found one! Cadance's horn glows blue, while her eye colour is purple. (Alicorns aren't disqualified from this, right?) I also noticed that Shining Armor's horn glows purple when his eye colour is blue... if you switched their eye colours around, it would match up. Might be something to do with their love for each other?

 

...Don't try and prove me wrong Flipturn. It won't end well, I can assure you. And if that is the case, then that is an adorable touch that the animators or whatever made!

[Blue Moon 6,341]

Hey, it's true! Confirmed it for the Flim Flam brothers:

 

 

Dang, I think I might have to check every single unicorn to find a discrepancy. I'll start with Sombra...

Ah, but wait, found one! Cadance's horn glows blue, while her eye colour is purple. (Alicorns aren't disqualified from this, right?) I also noticed that Shining Armor's horn glows purple when his eye colour is blue... if you switched their eye colours around, it would match up. Might be something to do with their love for each other?

 

Maybe it has something to do with their Cutie Mark Color instead. Twilight's aura is the same color as a part of her cutie mark. Rarity's is the same color as her cutie mark. Cadence's is the same color as her cutie mark. Flim and Flam's cutie marks both have green on them which is the color of their auras.

[Ampharos 1,009]

I for one don't think Twilight has a matching aura. Her eyes are like a deep lavenderish color while her magic is a fairly pink/magenta color. I'm sure there others that don't match, but Pumpkin Cake actually does...

 

[An Old Head 4,816]

Maybe it has something to do with their Cutie Mark Color instead. Twilight's aura is the same color as a part of her cutie mark. Rarity's is the same color as her cutie mark. Cadence's is the same color as her cutie mark. Flim and Flam's cutie marks both have green on them which is the color of their auras.

 

I dunno, that seems inaccurate. Pumpkin Cake and Sweetie Belle are both blank flanks, wouldn't that give blank flanks a hint as to what their cutie mark might be? Sweetie Belle would know that her cutie mark would have green on it.

 

Either way, Trixie's horn colour wouldn't match her cutie mark in that case. It's like a pinkish colour... her cutie mark is of a blue moon and blue star wand.

 

[Descant 1,897]

Well, originally a Unicorn's magic aura was originally supposed to match the color of their eyes. You'll see a major difference between the aura in, say, S1E5 than in the end of S3, even between the same pony. It's because they revamped how Aura worked halfway through the show. Somewhere around the end of S1, the beginning of S2, (I'm not sure precisely which episode) someone at Hasbro said "the aura should be less sparkly, and should resemble their cutie mark color rather than their eye color" and that's exactly the change that happened. 

Twilight's cutie mark is magenta, and her magic is magenta.
Celestia's cutie mark is yellow, and her magic is yellow.
Rarity's cutie mark is ice blue, and her magic is ice blue.
Cadence's cutie mark is cyan, and her magic is cyan.

Shining Armor's cutie mark is blue and magenta, and his magic is magenta.

It's not hereditary or ocular or anything. It's solely based on the cutie mark.
 

Of course there will be exceptions, like Trixie and Vinyl Scratch, but that's the general rule of magic aura.

Edited September 6, 2013 by Descant

[True Rarity 1,240]

Uh, what about Cadence and Celestia? Their eyes are purple or pink, while Cadence's magic is blue, and Celestia's is yellow.

 

And now, pink is not a shade of purple. Twilight's eyes aren't the same color as her magic.

 

Shining Armor's eyes are blue, while his magic is pink like Twilight's. Maybe it's a family thing.

 

 

Looky, Amethyst Star's eyes are purple while her magic is pink!

 

Oh, and what about these examiners?

 

Edited September 6, 2013 by True Rarity

[An Old Head 4,816]

Uh, what about Cadence and Celestia? Their eyes are purple or pink, while Cadence's magic is blue, and Celestia's is yellow.

 

And now, pink is not a shade of purple. Twilight's eyes aren't the same color as her magic.

 

Shining Armor's eyes are blue, while his magic is pink like Twilight's. Maybe it's a family thing.

 

Oh, and what about these examiners?

 

Didn't notice the examiners, that's another good find. I don't think magic aura is hereditary, though; Celestia and Luna are sisters, so they would've come from the same parents - their magic aura differs. Cadance, too, is Celestia's niece, and her aura is blue.

Edited September 6, 2013 by Flipturn

[True Rarity 1,240]

Didn't notice the examiners, that's another good find. I don't think magic aura is hereditary, though; Celestia and Luna are sisters, so they would've come from the same parents - their magic aura differs. Cadance, too, is Celestia's niece, and her aura is blue.

You have a point there, and actually after I made the post I was thinking... it's just something that looks good. I mean, Twilight has a pink streak in her mane pretty much exactly the same as her magic aura color.

 

Shining Armor's is just pink because Twilight's is, I think.

[Lateon 589]

I believe we've discussed this in another thread similar to this one. I'll provide my original thoughts, then my updated ones:

 

At first, I believed that the magical aura of a unicorn's horn when they did magic was based off of their cutie mark. This would line up with Twilight's, Shining Armor's, Rarity's, Cadence's, and Lyra Heartstrings' magic, etc. I went further to say that since Sweetie Belle's magic is green, that means that her cutie mark will have green in it somewhere.

 

But, there are other examples that debunk this theory. Vinyl Scratch, Luna, the examiners, Celestia a little bit...

 

So, in my opinion, the color of a unicorn's horn is just something that is aesthetically pleasing and builds off of their color scheme or personality or being. Celestia's is yellow because she is regent of the sun. Luna's is blue because she's, well, blue. Vinyl's is blue (?) because of her mane. The list goes on.

 

TL;DR - horn aura = matches color scheme or character in some way. Nothing more.

Edited September 8, 2013 by HandMan

[Kitty Cat Combo 2]

you're not wrong but you're not right sometimes it's the colour of their cutie mark (twilight, starlight, princess cadence) sometimes it's the colour of their eyes (sweetie belle, trixe, film and flam) sometimes it's both (twinkleshine, lyra, rarity) and I guess sometimes it can be completely random I guess

[SugarNote 2]

On 9/6/2013 at 2:34 PM, Blue Moon said:

Maybe it has something to do with their Cutie Mark Color instead. Twilight's aura is the same color as a part of her cutie mark. Rarity's is the same color as her cutie mark. Cadence's is the same color as her cutie mark. Flim and Flam's cutie marks both have green on them which is the color of their auras.

Yeah but Sugar belle's cutie mark is pink... 

[Blue Manestream 2]

On 2013-09-08 at 11:38 AM, Lateon said:

I believe we've discussed this in another thread similar to this one. I'll provide my original thoughts, then my updated ones:

 

At first, I believed that the magical aura of a unicorn's horn when they did magic was based off of their cutie mark. This would line up with Twilight's, Shining Armor's, Rarity's, Cadence's, and Lyra Heartstrings' magic, etc. I went further to say that since Sweetie Belle's magic is green, that means that her cutie mark will have green in it somewhere.

 

But, there are other examples that debunk this theory. Vinyl Scratch, Luna, the examiners, Celestia a little bit...

 

So, in my opinion, the color of a unicorn's horn is just something that is aesthetically pleasing and builds off of their color scheme or personality or being. Celestia's is yellow because she is regent of the sun. Luna's is blue because she's, well, blue. Vinyl's is blue (?) because of her mane. The list goes on.

 

TL;DR - horn aura = matches color scheme or character in some way. Nothing more.

Well actually if you look in the newer seasons Vinyl Scratches aura goes from blue to pink, and her eyes are pink when her glasses fall off in "Slice of Life". I could be wrong, this is just what I noticed throughout the seasons.

[Storm Fizz 1]

I think it’s either the color lf their eyes, or it’s the main color of their cutie mark. Starlight glimmer, for example. She has light purple eyes, but her magic aura is turquoise. Her cutie mark has turquoise swirls involved. The same thing for Cadence and Celestia. Princess Celestia’s magic aura is yellow, while her eyes are hot pink, but her cutie mark is a yellow sun. And Princess Cadence’s cutie mark is as we know the Crystal Heart, which is a turquoise blue-sh color.

[Fluttershutter 2,454]

Twilight's is more of a red than purple. In the first season it was clear.

The animators might use the eyes as a base but if they think something else will look better they won't hesitate to change it.

[Morbid Curiosity 11]

See, I know I’m SUPER late to this discussion but I have a headcanon about this :3

Unicorns with magic-based talents have magic colors based on their cutiemarks. Meanwhile unicorns who aren’t magic focused have eye color based auras. 
Twilight and Shining Armor both have pink magic, as Twilight’s talent is magic in general, whilst Shining Armor’s is protection magic. They both have the pink stars in their cutiemarks, which is where their aura colors come from. Starlight’s talent is also magic, so her magic aura is taken from the cyan magic swirls in her cutiemark. Sunburst’s talent is the knowledge of magic, which is still a form of magic-based talent, so he gets his yellow colored magic from the beams in his cutiemark.

Meanwhile Sweetie Belle has green magic and green eyes, because her talent is helping ponies with their talents. Her cutiemark has zero green within it, because it uses the pinks from each of the CMC’s manes. Trixie’s comes from her eyes, too, because her stage magic doesn’t require knowing hardcore spells. Flim and Flam have bright green magic that matches their eyes because apples aren’t known to symbolize magic within Equestria 😭

Of course there’s gonna be outliers, like Rarity! Oh.. Wait, her talent is sort of magic related.. That’s why her aura is taken from the lighter blue gems in her cutiemark, not the dark blue of her eyes! Her talent is gem magic, that’s how she earned her cutiemark as a filly and why she’s amazing with finding gems as an adult. She’s got a lesser magic talent, but it’s magic related nonetheless!

Please let me know what you think of this, as it’s honestly one of my favorite partially-backed-by-canon headcanons I have! I love talking about this subject so much!

[Pixi-Pie 58]

I've noticed that Unicorns either have magic based on their eye color (like Trixie, Luna, Flim and Flam) while others have it based on their cutie mark (like Twilight, Rarity, Cadence, and Celestia).