This script will dynamically override the settings for XCreds to set the login screen wallpaper on your M1 iMacs to the proper color "hello" wallpaper for that color of iMac.

M1iMacXCredsWallpaper.py

#!/usr/local/bin/managed_python3

import re
import plistlib
import subprocess

colors = {
    "1": "Grey",
    "3": "Yellow",
    "4": "Green",
    "5": "Blue",
    "6": "Red",
    "7": "Purple",
    "8": "Orange"
}

cmd = "/usr/libexec/remotectl dumpstate"
regexp = re.compile('(?<=DeviceEnclosureColor\s=>\s)\d')

result = subprocess.run(cmd, shell=True, capture_output=True, text=True)
color_id = regexp.search(result.stdout).group()
    
color = colors[color_id]
wallpaperpath=f"/System/Library/Desktop Pictures/hello {color}.heic"

output_pl = dict(
    loginWindowBackgroundImageURL = f"file://{wallpaperpath}"
)
print(plistlib.dumps(output_pl).decode())