A problem some Mac users have been experiencing with OS X Lion is an issue where their desktop backgrounds are showing as a bland gray color instead of the designated picture or pattern. This happens randomly after the systems have been woken from sleep, and when it occurs, the background will not change even if you manually try to reset it.
It appears that this occurrence is a problem with the management of multiple monitors and desktops, which in OS X is the responsibility of the Dock. When this problem happens, you can usually correct it by using Activity Monitor to force the Dock to quit and relaunch, which reloads the desktop and monitor setup configuration and properly displays the Dock.
Unfortunately there is no direct fix that works for everyone experiencing this issue, and while Apple is looking into the problem, it appears likely that a software patch will be required to fully address it; however, in the mean time if you are experiencing this problem you can try the following options to correct the problem or at least reduce its prevalence:
- Kill the Dock
If this issue only periodically happens, then you can give your system a virtual whack on the side by killing the Dock process using Activity Monitor as mentioned above, but you can also launch the Terminal and enter the command "killall Dock," which will do the same thing.
- Change display and background properties
If the problem here is being exacerbated by odd problems with your display settings, then adjusting your screen's resolution and other settings might help by rewriting the affected configuration files. Some people have noted a lessening of this issue after changing these settings.
- Invoke the log-in window
Since the issue here seems to be how the Dock loads and presents the Desktop, instead of forcing the Dock to reload you can try invoking the Login window if you have fast-user-switching enabled (set in the Users & Groups system preferences). By going to the log-in window, even though the Dock process for your account is still running you will clear what it is displaying and switching back should have it refresh the displayed desktop.
- Disable screensavers
This issue does seem to revolve a bit around the configuration of screensavers and display sleep modes on the system, and may be linked to these screen options being active when the system goes to sleep. You can try disabling both the screensavers and the option to sleep the display, and instead have the system immediately go to sleep after a period of time. You can set these options in the Energy Saver system preferences and in the Desktop & Screen Saver system preferences.
- Reset PRAM
The parameter RAM on systems contains some display-related settings, which might be interfering with the Dock's loading of the display configuration when the system wakens. Resetting the PRAM might help your situation, and since clearing its variables will not hurt your system, giving it a try might help. To reset the PRAM, reboot the system with the option-command-P-R keys all held down at once, and hold them until the system automatically reboots. Then release the keys and allow the system to boot normally.
- Clear system configuration files
The last option is to manually remove any Dock and Screen Saver-related configuration files that might be associated with this problem. To do this, go to the user library folder (press the Option key and select Library from the Finder's Go menu), and then locate the following files and move them to the trash, followed by logging out and logging back in to your system:
Keep in mind that by removing these files you will be reverting your Dock and Screen Saver settings to their defaults, so be sure you have an idea of what is in your Dock before continuing with this. One easy way to capture your Dock's current layout is to take a quick screen shot of the system by pressing Shift-Command-3 (the screenshot will appear on your desktop).