Creative Image Editing in Photoshop - Black and White in Photoshop CS4 and Photoshop Lightroom - page 241

Toning with Curves
Using an adjustment layer of Curves is another powerful way to add tonality
to a monochrome image. Any color imaginable can be achieved using Curves
simply by moving from the Master channel (RGB) into the individual channels
of Red, Green and Blue. Placing a point in the midtone value, highlight,
shadow, or any value on the curve and raising or lowering the curve will alter
the tone of the image. Here are the steps:
Step 1: Create an adjustment layer of Curves.
Step 2: Click on the Master channel (RGB) drop-down menu
and click into the individual channels of Red, Green and/or
Blue. Place a point in the midtone value of the curve to begin, and raise or
lower the curve to alter the tone of the image. For a browner sepia tone, as
in this example, you need to add red by raising the curve in the Red channel,
and simultaneously remove some of its complement color cyan by lowering
the curve in the Blue channel.
Step 3: Creating tonal adjustments with Curves is a sophisticated method that
requires a bit of control. Curves tend to shift the density of the overall image
in the process of adding tonality. To compensate for this, change the blending
mode of the adjustment at the top of the Layers palette layer to “Hue” or “Color”.