I'm going to try and summarize what I know. Some of this has already been pointed out, but I'm trying to consolidate here, so bear with me.
Youko's hair is naturally red in both Tokoyo (JK-land) and Hourai (Japan). It's a darker red in Japan/Hourai presumably due to taking on _some_ characteristics from her Hourai parents.
- Youko's mother wants her to dye her hair from red to black, so that Youko doesn't stand out. Her mother worries that the simple fact of having red hair will make people suspect Youko of being a delinquent and playing around. In the book version, there's a teacher who seems to actively hate Youko for this exact reason.
- Youko says she doesn't want to dye her hair because dyeing is against school rules. I.e. technically there's no difference between dyeing normal hair to red/green/purple vs. dyeing normally red hair to black. Whether this is the whole reason is unclear b/c we don't get any insight into Youko's thoughts during the conversation.
- Youko's dad doesn't make a direct appearance in the book, but it's clear from indirect references that he, possibly even more so than mom, doesn't want his daughter to stand out. The off-screen anime dad echoes this.
Kamui wrote:I think that Reckless Angel may have a reason. The sons are born with the features from both his/her parents, thus a son from JK would have the features from them. In the case of a Taika, when the Ranka settles into the womb of a japanese mom, it's take the features of his/her japanese parents. When he returns to JK, he returns to his JK family features.
(BTW, Kamui, I just _had_ to correct your typo of worm for womb. That was just bugging me way too much. ~_~;; )
This is my understanding, too, with one correction -- see below. Taika children will take on some of the characteristics of their Hourai parents b/c people in Hourai look like their parents. However, a number of the traits originally intended by Tentei may still show through. I'm assuming that Youko's red hair is a strong enough trait that it showed through in Hourai despite the "shell" of changes induced by being born in Hourai.
The novel indicates that in JK-land, unlike in Hourai, children do _not_ necessarily look like their parents. Roughly translated quote from "Tsuki no Kage" book 2, p.150 below. This is when Rakushun shows Youko a _riboku_.
(Youko) "Do children look like the parents?"
(Rakushun) "Children look like the parents? Why?"
Asked this, as if it were truly an strange thing, Youko smiled wryly. If a woman in the shape of a human could have a child in the shape of a rat, there was probably no genetic relationship between parent and child.
"Over there, parents and children resemble each other."
"Really? That's odd. Doesn't that make you feel weird, somehow?"
"Does it? I wonder."
"Doesn't it make you feel uneasy having similar-looking people all in the same house?"
"Come to think of it, you might be right."