I hate brutalist architecture, but love the Win2K design language, for what should be fairly obvious reasons. I interact with software. I don't interact with architecture beyond the visuals. It can look as shit as it wants, but if it gets out of my way and lets me do what I want to do, then it's great. It'd be nice if it look better, but I'll take function over form 10/10 times.

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.

Making the UI grey seems just as conducive to productivity as the raw concrete walls on a brutalist office building.

I'd suspect one of the big constraints was that you had to support basic VGA: 640x480 at 16 bold colours on a cheap CRT.

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.

If you were to quiz people who preferred the Windows 2000 look, you would more likely get detailed answers regarding the design language and functionality. If you were to quiz people who preferred the XP look, you would more likely get answers revolving around colors and shiny.