A very common issue, is that the white balance of a photo is incorrect and needs to be adjusted.
Lots of photo programs offer an option to correct this by enabling you to specifiy an area in the photo that should be white. The photo program then automatically calculates to what degree all the other colors in the photo deviate and the white balance is corrected.
A good example, which probably better explains this solution can be found in a help video from photoscape, which has the ability to correct the white balance: http://www.photoscape.org/ps/main/help_whitebalance.php