I worked at an Apple Authorized Service Center and had been doing service on the iMacs with slot loading optical drives (like the one pictured above). Whenever I would need to erase a hard drive and restore the operating system I noticed that the desktop wallpaper color matched the color of the case. So a Ruby iMac would get a Ruby colored desktop, and same with Sage green, Indigo blue, and so on. How did they pull this off?
Most iPhones since the 5S and 5C have used colour-matched wallpapers by default, too, but those are highly-integrated devices. Sorge says that virtually every interior component of the iMac G3 could be swapped and it would still know which colour to use for the wallpaper. I love details like these.