Getting the white balance, ambient light conditions or saturating tones wrong can cause unsightly colour casts that spoil an image.
It happens to all of us at one time or another, so don't trash the shot, there's a quick and easy solution available in Photoshop.
Open the image and click Image>Adjustment>Threshold.
Make sure the ‘Preview' box is checked.
Move the slider to the left until only a few black pixels remain. Hold the ‘shift' key down and left click the mouse on a group of the black pixels. A marker will appear that says 1.
Now move the slider to the right until only a few white pixels remain. Hold the shift key down and left click the mouse on a group of the white pixels. A marker will appear that says2.
Click ‘Cancel' in the Threshold window and the original image will return with two markers. If you don't see the markers, open View on the menu bar and click Extras and they will appear.
Click on the black eyedropper and then click on the marker next to 1.
Click on the white eyedropper and then click on the marker next to 2.
Now click on the middle eyedropper (grey) and click on any part of the image that should be grey. You can have a few goes until the desired effect is reached. In this case I chose the stone block just above the keystone of the main arch.
When you are satisfied with the colour of the image click ‘OK'.
Click on ‘View' and un-check ‘Extras' and the markers will disappear.