The colors of the rainbow (light spectrum) contains 7 colors.
Rainbow Dash's mane and tail contain only 6 colors:
Red, Orange, Yellow, Green, Blue, and Indigo.
So where's violet?
Rainbow Dash's eyes are colored violet to complete the light spectrum.
(A smart move on the creator's side)
This also leads to her cyan-colored body to fill in the color between green/blue.
You have to knows these things to be a true Rainbow Dash fan.
Anypony support my theory?