We studied this in our outfit. Systems diagrams are easier to digest when hue is the identifier instead of dots, hash marks, diagonals etc.
Not exactly on subject, but I recall the old method of updating the official aircraft gospel. Blue, yellow or green paper was used for supplementary data, revisions or to indicate differing sub-types in the fleet.
I'd be willing to bet Old Smokey remembers when the books were all printed in "Courier".
