The wallpaper is set instantly but if I recall correctly the color only changes after a reboot so the new settings are applied. One way to "fix" this would be to close everything, launch task manager, kill explorer and use the "New task"-button in task manager to re-launch explorer. Not absoluitely sure if doing this applies the new color and I would not recommend that option. A restart would be the safest and cleanest way.
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Mart

- Chuck Norris once sold ebay to ebay on ebay.