^lol why did i get the impression that japanese high school students had to wear long skirts XD I thought it was some strange strict rule telling them to wear their skirts a certain length. Also what about hair dye? is there a rule saying they can't dye it? I assumed that's the reason why none of the members hasnt done anything to their hair.
You maybe got that idea from thinking about European/American Catholic private schools? Schools have regulations about skirt-length, but in high school, that regulated length is pretty darn short! They're not actually supposed to wear their skirts at Sailor Moon height...
Some schools have strict rules about hair dye, some schools have loose rules about hair dye, some schools have no rules about hair dye... Recently several schools in Tokyo that didn't enforce their own rules about hair dye have claimed they're going to start enforcing it (as in, April 2013 is the beginning of the new push on tightening up those loose rules). Remains to be seen if they will.
In the case of MomoClo, it's probably management. In "Jimusho ni Motto Osaretai" Takagi claims that nobody cares what she does, but I don't really imagine Takagi wanting brown hair anyway, and DEFINITELY can't imagine Aarin dying. Her mother would probably die. [that's a pun, sorry]
From your old comment, it is true about long skirts meaning a delinquent student. Imagine Sailor Jupiter, her school uniform had a long skirt which signified her as a delinquent considering she got kicked out of her old school. But once they made the live action, they had to make the skirt shorter to the appropriate length, probably as long as the others. The rules would basically say the skirts should be above your knees but not too short. We're not talking about non-uniform schools which can become a little too much tbh.
Hair dye i'm not sure of but one J-Pop singer i know got into serious trouble and couldn't graduate because she didn't have her natural hair color. She had to dye it back or wear a wig so they would let her. I'm sure many schools are strict while others are concerned but don't enforce it well enough.
And basically, if the girls were to dye their hair, it would mostly be in the form of wigs. In "Onna Matsuri 2012" they had on removable extensions in their respective colors (Reni had white rather than purple, though and they took them off without a problem). In their Harajuku performance, they wore wigs that were ripped off later on. For UFI, Catherine and Gorina had blonde or brown hair which are wigs. Not to mention Aarin's mother being really careful about her daughter with the regulations so dyeing her hair is out of the question. Reni... I'm pretty sure she wouldn't dye her hair even if it's stated the others don't care what she does but the fans will be VERY concerned. I can never imagine her with different colored hair or any of the girls aside from wigs. I'm pretty sure management wouldn't allow any of them considering they're innocent enough to go by rules even though they are against it. (Jimusho ni Osaretai. Momoka and Reni were displeased with the company).
I'd say dying is out of the question simply because they have long black hair.
Most colors would require bleaching first, which really weakens hair, and it would take ages for a change to grow out. Even with just the braiding it's amazing how well they hold up.