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
Now click on the OK button.
Visit the GIMP web site here
The Photoshop Color Layer tool does the same thing, but with support for 16-bit images.
For instructions, click here.
For more technical info, click here.