@Background Pony #1BF6  
Yes, is a mistery, but i think PhysicRodrigo add her the color of Marina from Saban’s Little Mermaid.

No one else has noted this, but I will. Fluttershy’s tail is green. This deviates from all of the rest, whose tails either match their hair or their coat/skin. Twilight and RD’s tails match their coats or skins, while their fins have the color of their hair. Pinkie and Rare’s tails match their hair.
 
And AJ’s exclusively has her coat’s color, though both she and Fluttershy are divergent in that the former’s skin is less orange than her coat would be, and the latter simply doesn’t have anything green apart from the odd dress she wears. Also, these two wear it well; they own the mermaid look in such a manner the rest don’t quite.