"But Fluttershy! You have that modeling appointment with Hasbro tomorrow!"

"Oh, no! I forgot about that!"

"At least let me dye it pink again. M-Maybe nopony will notice…"

And that's the origin of that!

You know, it takes balls to do something as outlandish as dye your hair in multiple colors. Think of how long it must have taken for Flutters to work up the nerve to get it done. Think of how proud she must have been of herself to not be afraid of what others thought and to go forward and get that kickass hairdo that ponyville's most confient pony wears.

You really should have ignored your aesthetic taste and just let her have it Rarity.