Wait a second -- while your explanation very
completely explains HOW, it
does not explain WHY. Why did IBM go through all that trouble to turn
#6 brown?
My bet, at the time, it was popular to display little "landscapes" as
part of system demos. Lower third of the screen green (for grass)
upper two thirds blue (for sky) some kind of small building represented
in red (for bricks) and... a tree. You need brown for the trunk.