Hairstyles of the mane six:

 

Twilight Sparkle:

Sleek, thick black hair, which she occasionally wears in a bun. Has clip-on hair extensions representing her pink and light purple streaks, but probably finds them embarrassing, and only wears them on special occasions/when she's feeling happy.

 

Applejack:

Typical blonde beauty. Really likes ponytails since she can wear them while working.

 

Fluttershy:

Dyed hair pink, was naturally light brown. Despite wanting to love herself and her choices, she's still really embarrassed when she gets attention for it out in public.

 

Pinkie Pie:

Has the thickest, bouncy, curliest hair all over the place, and loves it! She'll go wild with all sorts of dye, but unlike Fluttershy, couldn't care less about what others think.

Rarity:

Wears the hijab most of the time, but when she's not, she'll usually put it in a sophisticated bun or ponytail.

 

Rainbow Dash:

Brownish-black hair. Has all sorts of colors dyed in.

More humanized background/fan made pony headcanons:

Raindrops is Japanese.

Daisy is from the US, Lily is Native Hawaiian, and Roseluck is part white (Canadian), part Romani. They're in polyamorous relationships with each other.

Button Mash is biracial [black American/white], with his Mom being a white South African.