What you said may be true today, Flynavy, but in the 40's there was much less thought put into advertising/image with regards to the livery. Most carriers simply left their planes in bare metal, with the carrier's name and logo placed on the aircraft in a modest manner. Yes, now the red tail for Northwest is its corporate image, and should they introduce a new scheme soon (as rumors have stated), they will keep their tails red. It is the origin of the red-tail scheme that I mentioned earlier. I wish I could remember the source of the story, so I could verify whether the visibility benefit was the reason behind the red tails, or if it is just an airline legend.
"So many planes; so little time..."