This technique produce the best results and is extremely fast and easy. Use this whenever possible for non-raw photos.
Download and save the following JPG file to your computer then open it inside Adobe Photoshop.
title="jpg" height="16" alt="jpg" src="/./mambots/editors/jce/jscripts/tiny_mce/plugins/filemanager/images/ext/jpg_small.gif" width="16" /> twilight 128.67 Kb (right-mouse click> Save As)

Add a Levels adjustment layer (Layer> New Adjustment Layer> Levels).
Tip: We use an adjustment layer instead of applying levels directly to the image to practice non-destructive image processing. This way, we always have a backup of the original image as a layer.

In the Levels tool, click on the Gray Point eyedropper. Your cursor should change into an eyedropper. With this tool, you can click on an area that's supposed to gray (50% gray for best results). For the image we're working on, we can simply select on any cement road that seems closest to 50% gray. Try clicking on a few other locations areas with cement to see which color you get. I had to try several times to find the best spot.

Rollover the image to see the before and after effect.
