Is this an error?

Mana is blue, HP is red

I like the idea that the horn is a focus. And Tempest is showing us just what unicorn magic without focus is. Also I like the idea she’s also just exceptionally powerful even among unicorns.

@Frustration in Excelsis
 
I agree %100, another thing I notice which may have to do with her magic being unstable is that it isn’t just blue, throughout the movie it is shown that her magic is more like a gradient rainbow that starts with blue
 
here are some pics of what I mean  
 
 

Her horn and magic are broken. It’s not much of a stretch to assume that that’s the source of the discrepancy.
 
Personally, I never really saw Tempest’s as magic in the sense that the other ‘corns have. She doesn’t get the stable, soft glow others have when using magic, she only has a fount of uncontrolled sparks. She can’t cast spells or even just pick things up, she only has a source of unshaped, volatile energy that she can turn on and off.
 
The impression I get is that what we see in the movie isn’t really Tempest’s magic, because she doesn’t have the ability to form and shape it properly anymore. It’s simply whatever source of energy unicorns normally use to power their magic, but unshaped and undirected. It’s not the same color as her old magic because it isn’t her old magic anymore.

Probably because of her horn breaking, the horn seems to serve as the focusing mechanism used to focus magic into various spells and telekinesis, I wouldn’t be surprised if that changes its color along the way like a colored lense. Take away the horn and all you have as raw unfocused magic randomly shooting out with nothing changing its color

She’s a brokicorn.

@TexasUberAlles  
Same, I used to be almost blonde, now it’s almost black.

My hair is a completely different color than it was when I was a toddler. It happens.

Maybe they’re not remembering correctly? The comics confirmed Tempest’s memory was biased.
 
Maybe this:  

@TheDrizzle404  
It could be that, explosive energy is quite potent. I wager she was a very powerful filly along the lines of Starlight Glimmer and Twilight Sparkle.

Raw arcane energy is usually blue. It changes colors depending on who or how it’s molded. In her case she lost the ability to mold it. It just explodes.
 
Or at least those are all the headcanons I just pulled out of my ass.

@Background Pony #B0D4  
^ This

Her magical aura started out purple, but after her horn was broken it started discharging sparks of blue energy.

Could also be psychological in nature. Emotions influence magic color.

Maybe it broke the red out of her magic? OR like Darth Shy said, Storm King magic?

Maybe the blue is a sign of Storm King enhancement?

Her horn is broken, wtf is wrong with you! :3