Here’s a quick and dirty way to get rid of color noise (that’s the nasty variety that most messes up the shadows) while keeping the detail—no fancy filters or plug-ins required. Duplicate your noisy background layer. Go to Filter > Blur > Gaussian Blur, and blur it until all the colors run together and you can’t really see the details anymore. Then switch the Blend Mode to Color. The result: Noise goes, detail stays.
(from December 2008 Picture Doctor)