FreeGEM is an open source version of Digital Research's GEM. GEM is primarily known as the native GUI of the Atari ST series - although it was also available for other systems at that time. It uses a dithered blue as background, the other colors are not actual choices but the two colors making up the dithered background.
Preview#7a7ad9
Floppy Disk
Hard Disk
Trash
All colors4 · click to preview
Dithered Blue (Averaged)
#5454d4
Dithered Blue (Gamma-Corrected)
#7a7ad9
Stone Gray
#a8a8a8
Blue
#0000ff
Dithered Blue (Gamma-Corrected)
The same dithered desktop, averaged in linear light and gamma-corrected: a lighter blue. The simpler pixel average is #5454d4.