In some cases you run a filter on your image but you want to tune the Luminosity and Colors independently. The DOP Split Luma Color script takes a layer and splits it into a Luminosity and a color component.
Free download from this page (see free scripts section).
Copy the DOP_SplitLumaColor.jsx file into the Photoshop/Presets/Scripts folder (CS3 or newer only, works also in 64 bit)
Select the top layer (no layer groups please) and run the script DOP_SplitLumaColor from File->Scripts. As a result you get a layer group like this:
You can now tune the Color and Luminosity layers to fine tune the colors and luminosity by changing the opacity.
We often use this script with filters that both change luminosity and colors (e.g. HDR filters).