Split toning

Split toning is an effect which has its origins in the days of film. It involves tinting the highlights in a black and white image one colour, and the shadows another colour. The best results are when you use opposite colours for each, such as yellow and blue or green and magenta. 

To create the effect in the darkroom: There are many different ways to tone and image in the darkroom. I think the more professional way to do this is with toners such as sulphide and selenium. I think the most common way of toning in the darkroom is with stuff like tea bags where you simply run the bag over the image. This gives an almost sepia effect.

To create the effect digitally: To create the effect in Photoshop, start by converting the image (if not already) to greyscale. Do this by using a black and white filter or by turning down the saturation of the image. To create a split tone, use a colour adjustment layer as it lets you isolate the highlights and shadows in the image and apply different colours to each. Adjust the colour sliders until you reach your preferred effect. 

Here are some examples of split toning: