Here's some helpful information about adjustment layers:
Adjustment layers affect all layers below but do not affect any layers above.
Bitmap, Indexed Color, and Multichannel modes do not support adjustment layers.
All files types that support layers allow you to save adjustment layers: Photoshop, TIF, EPS. JPEG does not, so if you are preparing a Web image using adjustment layers, save a copy in another file format.
All of the following can be applied as adjustment layers: Solid Color, Gradient, Pattern, Levels, Curves, Color Balance, Brightness/Contrast, Hue/Saturation, Selective Color, Channel Mixer, Gradient Map, Photo Filter, Invert, Threshold, Posterize. The different types of adjustment layers overlap somewhat. For example, Brightness/Contrast, Levels, and Curves are all used for tonal correction; they come with ascending capability and complexity. I'm going to concentrate on Curves and Hue/Saturation as my preferred methods of adjusting tone and color in an image. For the most part, I find these two powerhouses give me all the options I need.