Curves adjustment layer

Layer - New Layer

In the color picker, double click to define a new color, set value to 80 (20% grey), click ok.

Fill the entire layer with that color.

Change Mode to multiply if you want to darken, divide to lighten, or make one layer for each.

Layer - Mask - Add Layer Mask

Now you can adjust the opacity (under mode) or paint the layer mask black or white to get the dodging and burning results without degrading your image as much as constantly changing, editing, saving, editing, etc.

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Levels adjustment layer (one color at a time)

Basically the same thing, make the new layer a solid color, value at 100, but saturation at 100 and red, green, or blue at 255. To get yellow, magenta, or cyan, mix 255 red with 255 blue, etc.

Only use multiply with this as divide will quickly kill your image. If a lot of color balancing is required, the image may begin to get too dark. To fix this, simply create another curves adjustment layer, using divide with a variable opacity, to lighten it again.

Again, layer masks can be used to apply this to certain areas of the image.

While the same controls as curves may be obtained by the combination of opacity of the layer multiply and the opacity of the layer mask, I don't think curves adjustment layer is an appropriate title for the first tutorial.

Since I don't know how to delete comments, I'm just going to add here that I should have labled the first tutorial lighten adjustment layer and darken adjustment layer.