I used BeOS as a kid while playing with different operating systems. All of the windows were yellow colored, and you could irrevocably damage your system if you ever tried to change the color.
Not sure, but my guess would be that the yellow was a hardcoded constant in the code in a few places and modifying it involved opening up system files in a hex editor and doing some edits, amateurs + hex editors = bad times.
Holding down a key combination while opening the shell application's preferences menu allowed one to change the window decorations to Windows 3.1 and OS 8 styles, amongst others: http://www.eeggs.com/items/1034.html
Good times.