Open the image that you would like to edit in Photoshop.
Create a Hue/Saturation layer by selecting the New Adjustment layer option and then selecting the Hue/Saturation option.
If you are looking to achieve a natural effect, select the area based on colour. Using the edit drop down menu, choose the colours that you would like to keep and make their values -100. The colour that you would like to keep will need to be made 0.
In this example we have targeted the cyan, blue and green channels to remove the colour from the sky and mountain in the background.
Create a new layer, select Black and White. Click on OK without changing the settings. Select this layer and change the Blend Mode to Overlay.
Your end results should look like this.