@Background Human  
Red is the color of all emotion, including joy, love, and the thrill of victory. It’s the color of creative, artistic expression and the happiness that comes from doing what you want to do. While white is the color of the whole greater than the sum of its parts, red is the color of the feelings that make friendship worthwhile.
 
In terms of Elements of Harmony, Laughter’s an obvious candidate, and I generally associate Loyalty with it, in the sense of being true to those you care about because you want to, not out of an externally imposed obligation. Mind you, I’m not that familiar with the pony card game’s mechanical color pie, so I’m not sure how well the Elements map to it…
 
Sorry. I can talk about color philosophy for hours. And probably have, all told.

@FanOfMostEverything  
Now that you mention it, where the heck does red mana fit on the MLP color pie? Thematically I’d expect to see it in pink and blue, and maybe a little bit in white with the more passionate creative types. But I’m not sure how well the mechanics line up. (Then again, a lot of red mechanics don’t make sense in a game without violent combat.)

@FanOfMostEverything  
Do it. My Containment Priest would enjoy that.

Then pay a red mana to put that Troublemaker onto the battlefield with haste and sacrifice at the beginning of the next end step.
 
What?