lol Hal' Jordan hair changes color's. So did Genis' hair, his also changed colors back when he was in his red costume and still being called Genis-Vell and Captain Marvel. Um, Ultimate Thor's hair got burned off. Uh, lets see, who else, ummmm, oh yea Juggernaut's hair is always changing colors, sometimes he's blonde, sometimes he's a redhead, sometimes he's got brown hair. Uh, Magneto's hair changes alot too, sometimes he has long hair, sometimes he has a normal short haircut. Gladiator's hair also changed a few times, sometimes he has a huge mohawk, then other times he has a short one, then in the Thanos series a few years back his mohawk went into a ponytail-type thing.
Most of these have story reasons.
Hal Jordan was growing older and stressed and graying at the temples. It recently was retconned into being a physical manifestation of his Parallax infestation thing.
Genis' changed when he got cosmic awareness, I think.
Juggernaut's hair is mostly just inconsistency, back in the 60s and 70s when he first appeared, the inks and printers weren't as sophisticated, they may have originally wanted him to have red/auburn hair, but the printer messed up, etc. That's why Moon Knight, though always stated to be "jet and silver" appears white, the printers back then couldn't functionally make a silver color. In the 60s they had trouble printing grey tones, which is why after a couple issues the Hulk became green for no apparant reason (later retconned into being an alternate personality thing, etc.)
In Thanos, Gladiator had been in prison, I'm sure they don't allow him his daily allowance of a gallon of dippity-do, thus his shaggy appearance.
In the seventies, most black heroes had afros because the white writers and artists honestly didn't know any better. They were cashing in on the blaxploitation trend, and thought the ghetto fashion part of black culture represented the whole, when it clearly didn't. Christopher Priest (then Jim Owlsly) and "Doc" Bright made it a point to change Misty Knight's ridiculous afro into a sensibly hair style a young black woman might wear. The moment they were off the Cage/Fist title (where Misty and Colleen Wing regularly appeared) and Misty turned up elsewhere, she had the afro back.
That's not to say they were wrong in giving some characters an afro, it was fairly popular, and made a recent comeback of sorts, but to have Luke Cage, Photon, Misty, Black Goliath (to name a character black goliath in the first place is stupid), Black Panther and all the black people appearing in BP's title, Falcon, and more all have an afro of varying sizes turns the afro into a stereotype. Photon/Pulsar/Monica's look in the upcoming NextWave series is nice, she has a "big hair" hair style, but it's not just a formless afro, it's combed back, she has a band, etc. It looks real as does the new Misty Wing look.
Now, all the black male characters have shaved heads, which is getting annoying. Some characters can pull it off. It makes sense for T'Challa, he looked great in Priest's series with the shaved head and stache, it was a mean, cold, business look and he was fierce. Cage just looks like some drug pushing homeboy.
As for Wonder Man's hair, I think there was a lot of confusion in Avengers recently as to how old they should portray him. Some issues he had really grey toned hair, others it was jet black. Busiek made a few nuanced pushes to say that Simon was a veteran of the scene, and that his multiple deaths hadn't been all that becoming.
Some artists need a class on hair, they need to just sit back and look at the various types of hair. A black person's hair is by and large a lot different than a white or asian person's. Curly hair does not come in one succint style. White people can have afros too (King Buzzo from the Melvins). There are some textures that are unattainable in real life, but seem to be prevalant in comics (Norman Osbourne! what is that thing on his head? or Namor, who has this brick hard widows peak). Don't be afraid to make them a bit fantastic, stylized and out-of-this-world (yet still atainable with enough effort!) but keep it grounded, and avoid stereotyping hair.
This is a big post on a miniscule topic, but yeah.