Duotoning

Here's a very simple/basic version of working with Duotones in photoshop. Duotone works on grayscale (black and white) pictures but allows you to select colours so that instead of having back and grey shadows you are able to have pink or green or blues ones.


First open the image in photoshop and convert it to grayscale by clicking Image > Mode > Grayscale. This will convert your image to a black and white picture. 
Here is my original image:

Once you have created a grayscale image you can then click Image > Mode > Duotone. (Converting your image to grayscale allows the Duotone function to be unlocked.) Once you have selected Duotone you will see that under Duotone type, it is set to monotone - you can select this and change it to duotone. (There is also tritone and quadtone that you can experiment with.)

Once you select duotone - you will see two colour slots. You can change the colour slots to colours of your choice. Keeping the first colour black creates more realistic images whilst changing both colours can create cool effects. You can use this feature to create soft sepia colours or cooler sepia tones.

You can also play around with tritones and mixing the colours around. If you want to save it as a jpg or png file for web you will have to convert the image back to RGB mode by clicking Image > Mode > RGB Colour

Have fun in photoshop.