Often you'll want to make color adjustments to a page or panel. You may want to push the highlights or shadows, lighten mid-tones, or add a color cast. This is where Adjustment Layers make your job easy.
Ordinarily, when you make an adjustment, like applying Curves to an image, once you save and close that document, that change is permanent. You can't open it and undo the Curves adjustment you did earlier. You can only re-adjust the image.
Adjustment Layers allow you to apply Curves, Levels, Color Balance and more and also give you the ability to edit those changes any time. You can save and close the document, re-open it, and edit the Adjustment Layer, turn it off, delete it, make changes to it, and more.
So we start with our basic image, and click the Adjustment Layers icon in the Layer's Palette, selecting Curves.

Now we have the Curves dialog menu, with the Curves Display Options open, and Light selected. This means when we drag up on any part of the line in the histogram above, we'll be lightening the image. Always keep you Preview Check button selected so you can see the changes as you make them.

Pulling up from the middle results in lightening the mid-tones. Shadow and highlight areas are only affected a little, while the mid-tones are boosted dramatically.

By adding Points to the upper and lower section of the line first, then pulling up from the middle results in lightening the mid-tones without affecting the shadow and highlight areas. The points 'lock' your curves adjustment in place, making it easier to adjust the area(s) you want.

To brighten the highlights without affecting the mid-tones or shadows, lock down the middle and shadow areas on the line, pull the white slider to the left (under the histogram), then pull up in the top half.

To boost the shadows, do the opposite of what you did for hightlights. Lock down you mid-tones and highlights along the line, push the black slider in, and pull down in the lower part of the line as shown below. This is really the first adjustment I'll make to this image - boosting the shadows. I click OK, and the new Adjustment layer becomes active.

Now I want to pull back on the saturation a little - I create a new adjustment layer, this time selecting Hue/Saturation

I drag the Saturation slider back a little, and click 'OK.'

I want to cool off the image just a bit, and push the shadows to more blue. I create another adjustment layer, selecting Color Balance this time, and adjust the tone balance in the Shadows. However, the it adds a bit of a cast to the rest of the image - most noticeable in the neutral gray background plane. It's a little too blue now.

In the mid-tones, I pull the sliders to almost the opposite of what I did for the Shadows. This helps neutralize some of the blues in the background.

Another way to push colors in shadow regions is to use the values of the page/panel characters to make a layer mask. Below, you can see the Red Channel of the color Layer. From here, I copy the sphere, and past it to a new alpha channel

In the Green channel, I copy and paste the cube to the Alpha Channel I created above.

In the blue channel I copy and past the cone into the Alpha Channel. Lastly I copy and paste the backgrounds into that Alpha Channel.

Below is the Alpha Channel where everything was copied and pasted. I click the white eyedropper in the Curves dialog box, and then click on the background. This adjusts the Alpha channel so that the background (and anything else lighter than it) becomes white.

Next, I click the black eyedropper, and click the darkest area on the Alpha Channel. This pushes the range of darks, boosting the shadows.

Lastly, I pull up all the midtone and highlight areas, leaving me with just the shadows in the image.

I go back to the color Layer. I load the shadows I saved in the alpha channel, and inverse the selection. I create a new Adjustment Layer using Color Fill, selecting a color as shown. The selection I loaded automatically becomes a layer mask (you can see it in the little icon right beside the 'Color Fill' Layer in the Layers Pallette. You can see where all the shadows are really pushed to blue. But we're losing depth and detail in the shadows now, and the image looks flatter, and doesn't have a nice dark range we used to have.

I change the Mode of the color fill adjustment layer to Color Mode. This brings back the nice dark shadows, but they're a little too blue.

I pull back the opacity of the layer to about 40%. Now we have dark shadows with a nice bluish tint to them.

I find the shadows are just a little too dark, so I double click the first Layer adjustment I did, and pull up a bit on the original shadow adjustments I did

To make it easier to see the difference, and to manage the muiltple Adjustment Layers, I select them all, then select "New Group from Layers..." in the layers palette.

Name the new group....

And presto, they're all in a new group...

Easy to close up the folder to save room in your Layers Palette.

Below, the Before (top) and After (bottom) images.
