Rainbows, Elements of Harmony, and Sunset Shimmer

Season 1, Elements of Harmony fired against Nightmare Moon. How many colors does the rainbow have?

Six.

Season 2, Elements of Harmony fired against Discord. How many colors does the rainbow have?

Six.

Rainbow Rocks, the Spirits of Harmony invoke their inner magic without the Elements of Harmony to fire the original spell. How many colors does the rainbow have?

Seven!

What does this mean?

I don't know!

But I can speculate, right?

I think what happened is that Sunset Shimmer was supposed to be the original Spirit of Magic/Friendship, being Celestia's student a good indication. She was even capable of using the Element of Magic in Equestria Girls 1, however, thanks to the corruption in her heart, she turned into a demon.

Now that she got redeemed, and helped the other Spirits to restore their friendship after the Dazzlings ruined it, Harmony deemed her to be worthy to be the Spirit of Magic again. As a consequence, she became capable of participating in the the Elements of Harmony spell as a second Spirit of Magic, supercharging it and resulting in that huge friendship alicorn thing.

What do you think?

(This discussion is also posted in the Writer's group: http://www.fimfiction.net/group/50/the-writers-group/thread/135527/discussion-rainbows-elements-of-harmony-and-sunset-shimmer)