In this tutorial I will explain a technique often used in the printing industry, so most likely, you'll have seen this technique used on a photo in some kind of article or fashion magazine... It's called Duotones and allows the Photoshop user to replace a greyscale image with inks that can be modified. This adds a tone to the image. Now you could try to accomplish the same effect with the Hue/Saturation tool.. Good luck trying but it won't work in a million years!
1. Open your image, the image should be in Greyscale Mode, if it's still in RGB mode (or any other mode), go to Image > Mode > Grayscale. If you look in that same menu again, you'll see that Duotone is now available. In RGB mode, its unavailable
Go to Image > Mode > Duotone
2. Well, everything is up to you now. You have to decide if you want one ink (Monotone), two (Duotone), three (Tritone) or four (Quadtone). Then chose your colors, don't forget to check the preview option
Extar note: in the printing industry they only use Duotone, Tri & Quad are rarely used
3. If you click on the first square next to Ink #, you'll open a curve, similar to CTRL+M, the curves tool. The second square is where you chose your color. You could pick an RGB color but here we'll be using pantone colors (click on the Custom button in the Color Picker window, there's a red square on the image below to show where it is located)
No need to say that this technique is excellent for "people" photos / wallpapers
More Tutorials: 
