You can make separate layers for the luminosity (grayscale) and color image information in your photograph.
Then, you can make changes to one or the other.
For example, color noise can be reduced on the color layer, without affecting the luminosity of your photograph.
Be sure to check off as you go along.
1) Preserve your original file.
If you haven't already done so, go to Preserve Your Original File.
If you haven't already done so, go to Make a Background Copy.
If you have more layers than just the Background and Background copy layers, merge them into a composite layer.
Go to Composite Layer.
Make a copy of the merged layer, just as you would with the Background layer.
Then, make the luminosity layer or color layer using the merged layer and its copy.
If you're going to make both a luminosity layer and a color layer, make two of the merged layers.
And, make copies of them.
Use one set (Merged Luminosity and Merged Luminosity copy) to make the luminosity layer.
Use the other set (Merged Color and Merged Color copy) to make the color layer
Below, where Background and Background copy are written, use your set of merged layers instead.
The following methods were adapted from a tutorial by Richard Lynch, author of the Hidden Power of Photoshop Elements.
1) Change the blending mode of the Background copy layer from Normal to Luminosity.
Blending mode is located at the top of the Layers palette.
2) Select the Background layer.
3) Create a new layer above the Background layer by clicking the Create a new layer icon at the top of the Layers palette.
The layer will be called Layer 1 by default.
4) Fill Layer 1 with 50% gray.
Go to Edit > Fill Layer.
In the Fill Layer window, do the following.
a) In the Contents box, select 50% Gray.
b) In the Mode box, select Normal.
c) In the Opacity box, select 100%.
d) Deselect Preserve Transparency.
5) Select the Background copy layer.
6) Press Ctrl + e to merge the Background copy layer and Layer 1.
10) Change the blending mode of the merged layer from Normal to Luminosity.
11) Rename the layer as Luminosity.
You have separated the luminosity information from your photograph.
The Luminosity layer may not appear to be grayscale.
That's because the color of the Background layer is visible through the layer.
Deselect the Eye icon of the Background layer and any other layers below the Luminosity layer.
Or, you can block the layers below the Luminosity layer.
Make a blank layer below the Luminosity layer, and fill the blank layer with 50% gray.
Rename the layer as Block the Background.
Now, let's make a separate color layer.
1) Duplicate the Background layer.
Right click on the Background layer and select Duplicate Layer.
Or:
Click, hold, and drag the Background layer onto the Create a new layer icon.
Or:
Make sure the Background layer is active (highlighted), and press Ctrl + j.
2) Rename the new layer, Background copy, as Color.
3) Move the Color layer to the top of the stack.
4) Change the blending mode of the Color layer from Normal to Color.
You now have separated the color information of your photograph.
The Color layer may not appear any different than the photograph.
1) Deselect the Eye icon of the Luminosity layer if you made a Luminosity layer.
2) As described above, block the layers below the Luminosity layer.
Make a blank layer below the Luminosity layer, and fill the blank layer with 50% gray.
Rename the layer as Block the Background.
Create a Levels adjustment layer above the Background, Luminosity, and Color layers.
Don't make any adjustments, just click OK.
Once you've created all three Levels adjustment layers, double click on the Levels thumbnail in each layer to look at the layer's histogram.
Be sure to deselect the Eye icon of the layers that you're not examining.
You want to look at the histogram for each layer, not a histogram that's a mixture of several layers.
Also, change the Channel box in the Levels window from RGB to Red, Green, and Blue.
Go to Separate RGB Layers (Channel Mixer): Introduction.
If you haven't already done so, go to Saving Files.