Photography is all about lighting and it's important to learn what flattering light is. Once you do, you will find that the color temperature of that light makes a huge difference in the over all mood of that image. I can't tell you how many potentially great images that I see that were taken outside on a sunny day and the auto white balance makes everything blue, wrecking the image. Check out the video below where Jay P Morgan shows us how to set the white balance manually on a 5D Mark II. All you need is a White Balance Disc or a nifty White Balance Lens Cap.