Did you ever wonder why Rainbow Dash had a rainbow mane and nobody else did?
Maybe it never crossed your mind that a rainbow is more than a superficial trait.
A little while after Rainbow fails her Wonderbolt audition, her friends begin to notice that her iconic rainbow mane and cyan coat are literally leaking color, leaving behind a shallow, lifeless shell of what Rainbow Dash used to be.
Without the old Rainbow Dash, everything seems to be thrown out of balance in Equestria, and the elements of harmony can't function. Together, they attempt to bring back color to the one they care for he most.
Through medical study and the magic of friendship, Equestria will finally understand Rainbow Pegasus ponies, and the existence of rainbows entirely.