Add color quickly and easily to a grayscale image using
Photoshop's Variations command. First, convert your image from
grayscale to RGB or CMYK. Then select a specific area of the image. For
example, if you have a photograph of a tree, select only the trunk.
After making your selection choose Image > Adjust > Variations
and in the resulting dialog box make your color adjustments. Make sure
the Midtones option button is selected and then click on any of the
color preview boxes to add that color.