That was a joke tweet.

Purple is the colour of ionized air, so that may be a step.

@bronywonderbolt  
Her message was from years ago.

Are we really still bothering this woman over shit like this? I mean I get wanting to know out of curiosity but it’s not like she has any say over the continuity of the show.

@Background Pony #2C5C  
That cloud is basically a big orb of raw magic. Normally it’d return to her (as seen by a previous image of mine), but by removing it, she’s feeling the equivalent of casting a spell with that much energy. So by the end, her body basically thought she’d just been through a massive magic duel.

@ThePonyTheorist  
So, as a defense mechanism to avoid overheating and damage to the neural structure due to much anger?  
That wouldn’t explain why locking away the cloud in a jar reduce the vital life constants of Starlight. As if she was depleting her ATP reserves each time she removed that cloud.

@Background Pony #2C5C  
Hm… I now accept this :D
 
Well, it could just be that she had a crazy amount of energy due to her intense anger outside of her horn. Internal homeostatic mechanisms would prevent it getting that crazy when close to the horn, but when external, perhaps it no longer regulates the energy to a ‘ceiling’?
 
@Dustcan  
Eyes on the front page for a few more minutes. Another one in production.

@ThePonyTheorist  
Is specific only if it fully jump the full energy levels. But each level has different sublevels too. And oxygen has several (as opposed to helium or hydrogen). The electron travelling the different sublevels would cause the different wavelength of the residual radiation. Specially because it’s a “byproduct” of the real use of the energy, that is the “magic”.
 
But Starlight Glimmer laughts at those stupid limitations of the universe, as I said.

@Archonix  
Possible… It IS purple… Could be in a low concentration? We know the cloud’s probably polar (strong IM forces while not being solid), so how would this work? A neon-rich compound with some argon in it, perhaps? Would be some distortion from the other groups being present, but could work?
 
 
@Background Pony #2C5C  
But the colour that a molecule glows when excited is very specific to the molecule :/ It could be a ‘soup’ around the horn, I suppose… OR, a specific colour relevant to the gas their horns are emitting?
 
 
@Dustcan  
I’ve been planning on making youtube videos on pony theories for a while, then decided “eh, what the heck” and spammed them all here after years of accumulating ideas. I still haven’t gotten through all of the things i’ve thought of… I guess I could go for runes next…

I’m getting a kick out of all these uploads.

@Archonix  
I’d tell you all a chemistry joke, but all the good ones  

Argon.

Horns are basically antennas for the unicorns.  
The energy they use to levitate or cast spells cause also the air around them to charge up and discharge in a fluorescent manner. Each unicorn has a different horn structure and thus different energy potentials, leading to a different coloration of the “magic”.
 
Starlight is just so awesome, special and powerful that defies any pitiful laws of quantum physics and can make her magic do whatever it wants disregarding the space-time fabric and structure. Including purple lightning.  
Best Pony needs no explanation, she is Best pony for that.

@ThePonyTheorist  
See the rest of my theories for this to make sense… Maybe I should make a tag for them? XD I assume i’d have to ask the relevant forum area for a tag, right?

Also note that Neon would be unable to form such strong intermolecular bonds, unless it was in a compound…
 
Perhaps it’s some neon-containing compound?

If it IS Neon, there is definitely at least some electricity passing through there. Neon’s colourless on its own.