Well, most airlines don't buy their planes new. They either buy used or lease them from other airlines to get them cheaper. Well, in both cases, the planes come with the colors (also known as livery) of the old airline it belonged to.
Now, the new airline could paint the entire plane to it's own livery, but painting a plane is a process that can take months, and a plane that's stopped too long is a plane that isn't making money carrying passngers/cargo. So, to circumvent that, airlines will paint one part at a time, and run routine flights in between paint jobs. Think of it as you getting your car repainted, but you paint the hood first, drive it for a month, then pain the hood, drive it for another month, and so on and so forth. During this process, the planes look rather weird and out of place. You see a United plane with Qantas colors, silver American Airline planes with blue Southwest tails, etc...
Well, in one instance, the Colombian national carrier, Avianca, bought some 757's from small, privately owned North American Airlines. So, Avianca, proceeded to start the paint job into their own livery as usual.
...Of course, Avianca forgot just one little detail...
...They forgot about what North American Airlines had painted on their tail.
Sometimes, it's better to lose some cash and not lose your independence at the same time :)
