What is color correction ?
Color correction is the concept of altering the color balance of an image to achieve a desired effect. It is primarily used to set an emotional tone or convey important information.
You can read more at the Valve wikipedia Developer.valvesoftware.com
Today I learned about color correction and how to use it in Source games. I won't tell you how to do that, but I will show you some comparison photos from Portal. The first picture doesen't use color correction, while the second one does.
Of course, you can use that feature to change the appearance of any map. You can make it black-white, purple, yellow etc, you can invert the colors, reduce them and many other options, but I chose to make it look darker, reduce the colors a bit and some other minor stuff.