To begin with I transform my image into a black and white image with the tones/contrast that I want. Next I create a hue saturation layer, the 2 colours I believe work best are blue and red as in the mid tones where they blend they form a nice purple colour.
This next image shows the result after the first hue saturation layer has been applied. You have to make sure the colourize box is checked and then just pick a colour and saturation that you think suits, as this is done as an adjustment layer you don't have to worry too much about getting the precise colour yet as you can come back and make adjustments later.
Next you create your second hue/saturation layer above the previous one with the second colour you want.