Different colours and tones can help to instil images with subtle moods and can be used for expressive purposes. Toning has been used by photographers for a long time and can now be emulated in digital photography using software such as PhotoShop. Below are two images I toned copper and blue:

A duo tone is an image that is made up of just two colours and typically these colours are black and a spot colour. The Modes on PhotoShop allow the photographer to create one-colour (monotone), two-colour (duotone), three-colour and four-colour (quadtone) images. Below is my experimentation with Selenium Duo Tone: