Kenneth Lee Gallery
GIMP Colorize Tool
Step 1 - Make sure your image is RGB.
Select Image > Mode > RGB
Step 2 - Open up the Colorize Tool
Select Colors > 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.
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.
Now click on the OK button.
All Done!
Get GIMP 
Visit the GIMP web site here
Photoshop Users
The Photoshop provides several ways to do the same thing.
For instructions, see Toned Monochrome Images with Photoshop.
More Information
For more technical info, click here.