@TheAntiCat  
Is that a trick question? Dash is the only pony so far noted to have a wildly unusual appearance for a pony, mostly regarding her unusual rainbow hair.
 
Whether its important or not, Dash IS the only pony who is pointed out to have unusual physical traits with her rainbow colors. In all other ponies, there is usually a little to no coloration between relatives and no pony’s physical traits are even considered noteworthy in show.

@PonyPon  
look at them again, different shades and highlights of purple
 
@TheAntiCat  
“So Dash is the only exception? Your little world doesn’t allow other genetic abnormalities?”  
an exception is an exception because it is an exception  
Meaning we shouldn’t base our public theories based on that, as there is like a 99% chance of it not being true from the very getgo.  
Also look at the exception we got:  
One of the ponies of the elements of harmony; she can create a special move that hasn’t been done in so long it has become a legend; a rainbow is the symbol of the elements of harmony, just as a rainbow is the haircolor of Dash; Dash was the one who caused all the cutiemarks of the mane6 to appear; even ignoring the sonic rainboom, she is also the fastest pony in all of equestria; Her hair is completely unnatural even in this world of magic.  
now let’s compare that to the OTHER exception you think is valid:  
A blond handicapped pony
 
If you want to have a headcanon, that is fine, but it should stay where it belongs, in your head. If you make it public, which is also fine, but then be prepared for someone to call you out on the bullshit logic. We can’t have discussions about this show if everyone makes just shit up and thinks everyone should simply follow along with it.

@FilthyRichFan25
 
This was in a flashback.

Obviously she won Most Creative ’cause of that monster back wheel.

@meiyeezhu  
take a easy man, winning isn’t everything…

that’s derpy?  
how did she got turned into a filly???

DDDAAAAAAAMMM YOOOOUUUUU DERPY HOOVES!!!!!!!!!! I SHALL HAVE MY—
 
 
REVEEEEEEENNNNNNNNNNNNNGGGGGGGGGGGGGGEEEEEEEEEEEE!!!!  

This was the only time in the episode I really smiled. The rest sucked.

@Background Pony #EABE  
I know she probably wasn’t in every episode, hence why I said nearly.
 
But she was in that gag with the Christmas tree in the recent Hearth’s Warming Eve episode, and she was at the convention in last weekend’s episode.

To be honest I do like the look of that design quite a bit. Not saying this because Derpy, honest.
 
That said, cute happy Derpy is cute.

@Kibate  
What about Rarity and her Mom?

@Kibate  
So Dash is the only exception? Your little world doesn’t allow other genetic abnormalities?
 
You must be fun at parties.

@Bry7x7x7  
When will this “they have the same mane color” finally stop? Mane color is NOT a genetic trait in Equestria(at the very least not parent-child genetic, there is still the possibility of recessive traits like what happened with the cakes, but that’s another story)
 
The only exception is RainbowDash, and her mane obviously is not “normal”, even in this setting

@Reb4Ham  
Doubtful. Probably just fans.

@PacificGreen  
that’s debatable. She hasn’t been in every episode. though she has made quite a few spotlight moments recently.

Seems like they’re putting Derpy in nearly every episode now. And in scenes where she’s the focus of a shot, too.

@Reb4Ham  
Come to think of it, the stallion has the same color mane as her.

@Reb4Ham  
Beats me

Are those by any chance Derpy’s parents/relatives?