Fei this, Fei that

In the cutscene at the end of Disc 1 where we see Weltall transform into Weltall-Id, its color changes smoothly from black to red; similarly, Fei transforming into Id changes his hair from black to red.

I see this not as a change in reflective color (i.e. what we see depending on which wavelengths of light bounce off or are absorbed by an object, which is what paint or hair dye changes), but as an emissive change consistent with blackbody radiation. Essentially, Weltall-Id is glowing like a heating element (I'd put it at around 1000K) due to the power that Id is putting through it. [supporting citation: https://www.xenoserieswiki.org/wiki/Weltall-Id#Description ] (This is also around the same temperature of the lava flows around the Zohar at Bethlehem, hence the same color)

I think this also applies to Id's hair. My impression is that Id doesn't have full control of the Contact's power, and so it leaks out of him, essentially heating up his hair, and giving it the same blackbody glow as with his Gear.

The pink color could be explained if Fei's hair is naturally very reflective (say, as a very shiny black), and so under a strong white light like sunlight, the emitted light and reflective light combine for a pink overall color. In a dark room, all you'd see is the orangish red glow.

As a fully realized Contact after his conversation with the WE, Fei has full control over that power; basically his blackbody spectrum crosses what powerfologists call the Bishonen Line, and so his emissive spectrum is much more human. He's no longer leaking power out through his hair, thus it returns to its original black.

Xenogears is an expression of that fully realized direct-from-the-WE Contact power, and so its white and blue color scheme is closer to a temperature of 6500-7500K. (The Sun, for reference, has a photosphere (surface) temperature of 5773K.) This would put it in class F to low A on the Harvard classification. (the Sun is G2)

According to the recent new (superior) Vector Translations translation of PW (as opposed to the old 2008 UltimateGraphics scanlation you see everywhere that UG did while they were learning JP; a translation that has many many errors), W-Id glows red because "about two-thirds of the generated heat is released as electromagnetic waves" through its outer frame, aka its exoskeleton. Quote: "the electromagnetic wave spectrum radiated from this cooling system also contains wavelengths in the visible region, and this is the reason for the machine's red-hot glow, the wing-shaped light generated from its back, and the ambient glow that envelops the machine."

Back to the hair— we do see Id, Elly, and Myyah glow with an ether-related energy, but that's full body and only a temporary thing. And Elly's is a white-blue, but we don't see her hair turning blue so idk (or well, green? a mix between her hair color and that blue)

(blue, Khan also glows a deep blue in the animated cutscene, someone who doesn't have any strong connection to the Zohar; though those red/blue glows in that animation could very well have been just an indication that what Fei was seeing was not real, something I think I've seen done in old anime [or perhaps a blue/red good guy/bad guy distinction for the viewer, idk] bc that scene was originally intended to be used when Fei was coming down from Citan's house and seeing a hallucination on the mountain path rather than what they ended up using it for in the final build of the game: a dream he had on the sand cruiser).

But yeah, nothing like W-Id's electromagnetic glow, as you can see Id temporarily glowing with that red/pink full body glow while W-Id directly behind him isn't. So like, is it just a game development thing where they just don't add it or would adding that same glow to W-Id all the time be too hard on the PS1? idk. PW *does* additionally state "and the ambient glow that envelops the machine". 🤔🤷‍♀️

Anywho, as unsure of that explanation as I am based on all of that (while even Takahashi has said he has no idea why Id's hair/eyes/skin changes color and clothes change), an explanation like yours is probably the closest one we'll ever get to the hair question 😂

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Unrelated, but I also couldn't help but notice the Xeno Series wiki also says "Unlike Alpha Weltall it [Weltall-Id] can recycle energy", which is incorrect. According to the new VT translation, quote: "Unlike O.R.W., where over 99% of the generated heat is recycled as energy for machine operation through a self-contained circulatory system, cooling was extremely difficult for Weltall, which is constructed with standard equipment."

The Xeno Series wiki appears to be using the old bad translation, as after seeing a good translation of that paragraph you can clearly see how it was misread by someone who was mid-learning Japanese as they were translating the book.

It is an Omnigear, after all. Weltall-Id is an imperfect replica of OR Weltall, not the other way around; it doesn't do anything better than O.R.W. A Gear-asa could never be on the same level as a Gear-bara. W-Id gets close, in that its Slave Generator is on par with O.R.W's, but its frame(s) were never made for that heat, leading to it's scant *10 minutes* of operation time 😂💀, only a Gear-bara aka Omnigear aka God Gear could handle that. No matter how much they (Grahf/Solaris/Kislev) haphazardly threw together several semi-solutions to try and make up for its overheating problem at the end of its production.

(and no, you're right, its definitely not Earth lol)

edit: yeah that page is entirely taking from that old bad translation. AND also misreading the parts that that old bad translation gets right! 😂😭 New translation on the left, old on the right. (oh no, even the "magnetic refrigeration" part! I just read a recent fanfic that was clearly taking from this translation as well and getting a completely different idea of how W-Id cools from this 💀)