For this tutorial I'm going to use Alex from Atreyu because he's so sexy like that.

1. Open up the picture you are going to use.

 
2. Then go to your menu bar and select Colors.
   
    

3. Under Colors, select Colorize.
   
    

4. It should take you to a window that looks like this.
   
    

5. Fiddle around with the numbers until you get the color that you want.
    

6. Voila! You're finished.
   

For the faster version...

1. Open up your image.
    
2. Press Shift + L.
    
3. Follow steps 4-6.