Without some fiddling, the background on the external display defaults to Apple’s dark linen background on the desktop. If I try to change it I can see the changes reflected through the translucent menubar, but the desktop background stubbornly hangs on to that dark linen pattern.
After the obligatory 30 to 120 seconds of research on Google, it’s easy to learn this is a common problem. I discovered a fix of sorts.
The linen pattern is associated with fullscreen apps. I went through my apps and pulled them out of fullscreen mode. I put them back in fullscreen mode and my preferred desktop background remained.
Interesting point of note: I “minimized” the fullscreen apps Safari, iTunes, and third-party email client Sparrow to uncover the correct desktop image; however, Terminal.app remained in fullscreen mode as my background returned to normal. Maybe it’s a display bug with Safari or iTunes? “So…”1 I exclude Sparrow because, as a third-party app, I assume the widespread reports of display weirdness are associated with stock applications.
N.B.: Learn how to type weirdo characters like “œ” at How to Type.net