Have you ever taken a picture and asked yourself, "Why is there a blue cast on this image.  There wasn't any blue in the image?"  Well the first thing I would look at is your white balance.  What is white balance?  It's the way your camera sees white tones in the image.  Most people grasp the idea that light can have different colors and that light helps reflect colors in an image.  So light of different colors can reflect different casts on the image and can effect the color reflections of the objects in the image.  Therefore, if light has a blue tint to it then the objects in the image will reflect that blue tint.

So you may say, "Well my subject has a white blouse and  I see it as white.  Why does my image show it as a light gray?"  This is because the white balance in your camera is incorrect.  If you use the Auto White Balance setting on your camera it will look at the entire scene and expose your image based on that.  It also uses a benchmark to change pure white to 18% gray and adjusts the other colors in the image accordingly.  "What", you say.  "Why 18% gray?  Why not 100% white?"  This is because 18% gray is an average of the white values in a scene that are perfectly exposed and perfectly lit. Even in scenes where the light has different colors, your Auto White Balance setting will try to expose pure white as 18% gray.  This is also why so many winter images show the snow with a gray tint. 

Here's a before and after. Notice the Red tint before correcting the white balance.

So how do you fix this.  First, you need to realize what your camera is going to do and respond accordingly.  Understanding what the camera is doing is the first step in determining a proper way to resolve the issue.  There are several ways to approach the issue.  One way is to continue shooting with Auto White Balance and fix all of the images in the computer.  The obvious problem with this is the time involved in getting the image(s) right along with the possible color inconsistency of images in a series.  A better option would be to use one of the presets on your camera.  So if you're shooting in shade, use the preset designed for shade.  The problem with this solution is that you're still relying on your camera to determine white balance.  It is still looking to average your image to 18% gray but making adjustments based on the preset.  So what should you do?

The best way to fix this issue is determine what the correct white balance is and apply it to your image.  There are several ways to do this but I will focus on three different approaches.  Two of these involve setting a custom white balance in your camera so that as each image exposed will reflect the correct white balance.  The method that goes back to the film days involves shooting an 18% gray card to set a custom white balance.  Different cameras use different steps to setting a custom white balance, but essentially this means shooting something that you want the camera to model it's white balance from.  By shooting this gray card you are telling the camera what to use as a reference for 18% gray when determining exposure.  Once this is done you can shoot your images with the proper white balance.  That is until the color of light changes (ex. moving from sun to shade, or from outdoors to inside).  When this occurs you must repeat the process of establishing your custom white balance.  There are many resources for how to do this properly so I will not cover them here.

The second method, and the one I have recently begun to use, is setting the custom white balance using a filter over the lens that is calibrated to 18% gray and shooting into the light source.  The tool I use for this is the ExpoDisc by Expo Imaging.  I think it's much easier to use than the gray card and for me it's more accurate.  As with the gray card, once the custom white balance is set in the camera you can begin to shoot all of your images with the proper white balance.

Finally, much like the gray card you can shoot the ColorChecker Passport card by Xrite.  This is a little checkerboard of multiple colored squares.  It actually does two things.  It sets a reference for use in post processing for both white balance and color balance.  During post processing you set your white balance based on the white square in the checkerboard.  This is your reference for what is truly white.  You can also use the entire checkerboard to create a profile for your camera in that particular light.  This profile is then used in the software during post processing to properly adjust exposure.  It's much like the presets for setting white balance.  This process works very well, but again you must apply the corrections during your post processing. 

Any of these will work to help you get the color right.  I'm not really advocating any one of them.  You should select the one that works the best for you.  The point is to use something.  Nothing screams AMATEUR more than incorrect white balance.

Hope this helps.  Happy shooting!

Here's another example of the difference between an incorrect white balance and a correct one.