Creative Image Editing in Photoshop - Black and White in Photoshop CS4 and Photoshop Lightroom - page 230

After
Hand Color Black and White
There are a number of techniques that can be used on black and white images
to give them an interesting twist, and adding hand coloring is just one such
method. By using the old fashioned Marshall Oil hand colored look you can give
new life and vibrancy to a black and white image. In order to do this technique
you will need to be relatively familiar with layers, selections, brushes and the
color picker, although all of these tools are covered briel y here.
The Method
Step 1: Open
Open a black and white or toned image.
Step 2: Change the mode to RGB
Make sure that the Image mode is RGB Image Mode RGB color in order
for the document to be able to receive and display color values.