[hakfoo]

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.