Move your cursor over the image on your left to see the photo after touchup.

This photo was taken in a room illuminated with a tungsten light source. This type of light frequently causes a yellowish tint to the image. This tint (color cast) is obvious here as the towel, that I knew to be white, is obviously not white in this photo. You can almost always spot a color cast by looking at some part of your photo that you know should be a neutral color be it black, grey or white.

Picmonkey is a free photo editor that does a great job of taking care of a color cast problem. Check out the tutorial at: Removing a color cast with Picmonkey.