I was young at the time and this seemed absurd to me. Why would you willingly use a UI that looks like wearing an old grey tie for a dusty office job in a depressing concrete building?
There is a German saying "Grün und blau schmückt die Sau" ~"green and blue grace the pig" - and XP was green and blue and shiny like hard candy.
I use Linux by the way... but not all Windowses look ugly IMO.
I think is so ugly that I'm half tempted to chatgpt fact check this comment. It very well could have been a deliberate conspiracy to lie to the public and say it looks good to make some austerity go down easier.
The only reason brutalist buildings were made in that style is because it's cheap(er), and because some architects started to smell their own farts a bit too much. Win9X/Win2K were made the way they are because it's actually conducive to get shit done.
With that palette, using white/black/dark grey/light grey for the UI leaves most of the "saturated" colours for the software's own messaging. If you make the main body of the UI red or yellow, it's going to wash out if software uses those colours for alert indicators. If you make it blue, it's probably going to be a bit subjectively dark, and you'd need to use light characters for contrast. White input fields might have a skeumorphic aspect in that they look like paper you can write on.
An interesting comparison is the colour schemes of CDE. It was usually used on 256+ colour systems, so many of the themes are pastel-centric, but it's aggressively beveled and has similar high legibility.