Kenneth Morris Lee
GIMP Colorize Tool

Step 1 - Make sure your image is RGB.

Select Image > Mode > RGB

GIMP Colorize

Step 2 - Open up the Colorize Tool

Select Colors > Colorize

GIMP Colorize

Step 3 - Choose your color

Specify Hue to get the color you want.
Specify Saturation to get the intensity of color.
Leave Lightness at 0 to preserve the tonal scale.
Toggle the Preview option to compare with original.

GIMP Colorize

Now click on the OK button.

All Done!


GIMP Colorize

Get GIMP 

Visit the GIMP web site here

Photoshop Users

The Photoshop Color Layer tool does the same thing, but with support for 16-bit images.
For instructions, click here.

More Information

For more technical info, click here.