All Photoshop color adjustment tools work essentially the same way; they map an existing range of pixel values to a new range of values. The difference between the tools is the amount of control they provide. For an overview of the color adjustment tools, see Color adjustment commands.
There are two ways to adjust the colors in an image. The first is to choose a command from the Image > Adjustments submenu. This method permanently alters the pixels in the active layer.
The second and more flexible method is to use an adjustment layer. Adjustment layers let you experiment with color and tonal adjustments without permanently modifying the pixels in the image. The color and tonal changes reside within the adjustment layer, which acts as a veil through which the underlying image layers appear.