The Colorize tool renders the active layer or selection into a greyscale image seen through a colored glass. See HSV Color Model for Hue, Saturation, Luminosity.
You can get to the Hue-Saturation tool in two ways :
In the the image-menu through: → →
          By clicking the tool icon:
          
             in Toolbox, provided that you have installed color tools in Toolbox.
          For this, go to
          Tools Dialog.
          
          in Toolbox, provided that you have installed color tools in Toolbox.
          For this, go to
          Tools Dialog.
        
The slider and the input box allows you to select a hue in the HSV color circle (0 - 360).
The slider and the input box allows you to select a saturation: 0 through 100.
The slider and the input box allows you to select a value (luminosity): 0 through 100.
The Preview button makes all changes dynamically so that they can be viewed immediately.