White balance is the color balance of your photo. When your digital camera looks at a scene, it wants to make everything 18% gray. Photos of snow can turn out gray and photos of black animals can turn out gray.
So what can we do to ensure the colors come out as they actually were? For starters, shoot in RAW instead of JPG. You have more control later if you shoot RAW. Next you could take a reference shot with a gray card to be used to set the white balance during the post processing phase or you can use the gray card shot to set a custom white balance in the camera.
To be honest, the modern cameras are good enough that most of the time the colors are close enough to live with. But if you need/want to ensure accuracy in your colors, you have to take matters into your own hands.
Figure 1, figure 2, and figure 3 show the image the way the camera interpreted the scene on the left and the corrected version on the right.
You can purchase a small reference device like these in figure 4. One is a 12" x 12" fabric white balance reference, one is a wallet card, and one is the Spydercube. Any of them will work. The Spydercube lets you set absolute black and specular highlights that's another expensive model. I started with the 12" x 12" model. I trust the Spydercube to be the most accurate 18% gray but will often use the wallet card, which I trust the least, in a pinch.
A note about color
You cannot rely on your monitor accurately reproduce colors in your images. In fact, I would say you can count on it not reproducing your colors accurately. If you want your monitor to accurately reproduce the colors in your images you will have to calibrate it. You can get one of these reference cards and compare what is on your screen to an image you take of the card or you can get a piece of hardware that does the calibration for you. One such piece of hardware is the Spyder from Datacolor. The hardware calibration solution is not cheap but if you want to post process for printing might be worth the investment.
If you want to print using a print shop you will also need to order proof prints from the printer to compare what you see on your monitor to what they print of those images.
Setting Custom White Balance In Camera
Using this technique you set the white balance in the camera and all shots in the same lighting will have the correct white balance. If you use this technique you will not have to worry about setting the white balance in post processing.
NOTE: The menu screens are from a Canon 6d. You may need to refer to your owners manual.
I find that I have to use this technique when using several neutral density filters to darken the scene because the shot turns out red.
1. The first shot shows a white balance that is off, by a lot, because of the use of neutral density filters. (figure 5)
2. Here I have turned the camera away from the sunset to take a shot of the gray card. If I didn't turn away from the sun, the lighting on the gray card would have been wrong. The gray card doesn't have to be in focus for this to work. (figure 6)
3. Next, go into your menu and select Custom White Balance and select the image with the gray card. This selects the image to use for the custom white balance in the next step. (figure 7)
4. Then change your white balance to Custom (figure 8)
5. Now take your shot using the custom white balance (figure 9)
Setting White Balance From a Reference Shot
In this instance I took two shots of the septarian nodule. One with my Spydercube and one without in the same lighting. I could have propped my wallet gray card against the stone as well. I have imported both images into Lightroom and no edits have been performed. We will set the white balance in the reference shot and copy it to the other shot.
1. Open the reference shot in Lightroom and open the Develop module (figure 10)
2. In the basic panel select the white balance picker tool (figure 11)
3. Select the grey area of the reference shot to set the white balance (figure 12)
4. Here you see the color corrected image (figure 13)
5. Select Settings, Copy Settings (figure 14)
6. Select only White Balance and click Copy (figure 15)
7. Open the image without the reference gray card and enter the develop module (figure 16)
8. Select Settings, Paste Settings (figure 17)
9. Now you see the second image color corrected with the white balance information from the first image (figure 18)