changing desktop wallpaper on the 2d external monitor on Lion - how?
 

I've got Lion 10.7.2 and two external monitors connected to a MacBook Pro. I'm able to change desktop wallpapers on the laptop and the primary monitor, but not on the secondary monitor. Mac OS's Secondary Desktop dialog window shows up, but selecting an alternative image or pattern for the desktop does nothing on the 2d monitor (it works on laptop and the 1st monitor).

Is there known way around that?
Even changing the color from default light grey to something darker would be an OK temp solution.