Selective Desaturation

Palm Tree and Skyline
Rome, Italy

Canon Rebel XT
Focal Length 21mm
Aperture 5
Shutter 30 sec.
ISO 100

Given below is the same photo with no adjustment made to the red color channel.  I wanted to achieve a darker, richer look in the photo and remove the red cast that was present due to the street lighting.  Color casts such as this are common with sodium vapor lights.  After reducing the red channel by around 80% I adjusted the "black point" in aperture 2.0 to give the sky a richer, darker look.  For both of these photos I also lowered the yellow levels.  I also lowered the luminosity of the greens and adjusted the definition by 100%.

And below is the untouched original.