Did you notice how deep the imagery goes? Those ponies are like avatars of their elements.
Pinkie's hair is round and swirly, any deviation from the norm won't even make a difference. Any child could draw those endless bends beautifully. Totally Pinkie.
Rarity's hair is exquisite - a real challenge, and artists can get frustrated trying to draw it properly. Totally a Rarity, requiring high skill.
Rainbow Dash wouldn't be satisfied with just some color. She wants the whole color spectrum in there, to stand out, to make people gaze in awe like watching a rainbow.
Applejack's hair is practical and simple, like a bundle of hay, and even colored in that style. Condensed so that it doesn't get in the way during work, yet allowing people to see its fullness.
Twilight Sparkle's hair signals a special snowflake with those streaks, haha. They're a pony's rallye stripes.
Fluttershy's hair flows like water, not resisting things, finding its way passively, just like her personality. The color is rose, basically pastel pink, and pink is often associated with the heart, which is what she'll melt when she looks at you. Her mane even forms a heart!