Monterey Login Screen is Stuck on Chroma Red (different than the Pink Monterey Valley screen!)

I've been looking into this quite a bit. Turns out, this is not a bug. I'm not sure if it's all M1 Macs or just the new M1 MBPs, but I can at least now confirm that all M1 MBPs show Chroma Red/Chrome Blue by design.

A LOT has changed in macOS since the Intel Catalina/Pre-Catalina days and M1/Monterey. I did not realize that the macOS I knew is long gone. I learned quite a bit by reading through the articles here: https://eclecticlight.co/m1-macs/ The author, Howard Oakley, has done an absolutely incredible job documenting the changes that have been made to macOS in the last 4-5 years.

These wallpapers are either part of the SSV (Sealed Snapshot Volume) or part of the new iBoot on M1 Macs—I'm not sure which. In any event they are (practically speaking) unchangeable. They are part of the OS that is signed and distributed by Apple and can be (and likely are) model number specific. I say that because that's how it's been on iOS for a while, it's just less noticeable—iOS default wallpapers are different based on model.

The reason behind locking down this wallpaper is security. All parts of the system that load before login are designed to be signed and bit-for-bit unmodified by Apple. How/why Apple allows the user profile icons to load I'm not sure. If you notice, they are extremely low resolution before you login. If I had to guess what Apple is doing here is saying that user profile photos are okay to load because they are heavily compressed, so they may feel there isn't a practical way to include a malicious payload into a user profile picture that is only a few kilobytes, so it is not deemed a security risk. A high resolution desktop wallpaper, on the other hand, could be a security risk so it is not allowed; and the alliterative—compressing a desktop wallpaper to a few kilobytes would be quite ugly. How and why they decided to make it Chroma Red/Chrome Blue I have no idea, but it is what it is.

One final note: it might be technically possible to change this, but to do so you would have to turn off almost all of the security features and really dig around to find out where this screen is. You'd also have to enable Permissive Security mode and turn off SIP. The wallpaper would also be reset with any future macOS update. It may cause all sorts of other unintended problems, since system integrity will no longer be bit-for-bit verified.