This is Rainbow Dash:
I somehow don't like this design, because this suggests that RD dyes streaks in her mane, which seems to me contrary to her character traits.
Which is why I like this fan design:
This hints these are not dyed streaks. Rather, her mane, apparently naturally, shines in all colors of the rainbow.
Is it possible that her mane acts like a prism, dispersing the light and producing this rainbow effect? Or is it possible that her hair gathers moisture from air and the water droplets produce a literal rainbow?
The latter option would be especially nice, because perhaps, once her hair gathered enough moisture, it would allow RD to perform such a trick:
If this is impossible, what would be the required changes to how the nature is wired to make this possible?