"Real Color", as we humans see it, is the light that is reflected from an object. There are sources of light: such as the sun, the moon, artificial lighting, a camera flash, etc. This light strikes an object and it is reflected back to our eyes, which record it and pass it on to the brain, which interprets it for us. An apple looks red, or green, or yellow, because the chemicals in the skin of the apple reflect back these wavelengths. But the "true color" of an object changes as the subtle color tone of the light that shines upon it changes.
For example: We see a piece of white paper in an office that is illuminated with florescent lighting. The paper looks white to us. We take the paper outside into the bright sun, it still looks white. We take the paper home with us and look at it under the tungsten lights that illuminate our house; it's white. We take it into the shade and it still looks white to us. It's not really pure white in each of these situations. Our brain knows that this paper should be white so it does some fancy interpolating for us. In reality, the paper in the office probably had a bluish hue to it, as it did in the shade too, and it had a reddish hue in our home, and was maybe slightly yellow in the sun. Our eyes simply adjust and send the right "white balance" to our brains. Cameras do not have the wonderful flexibility of the human brain. Although there is some great technology out there to make these machines produce the color that we SHOULD be seeing, by correcting the WHITE BALANCE of an image as the image is captured or processed, most of them are not 100% perfect all of the time. There will be times when an image has a COLOR CAST to it, maybe a slight magenta, a bit too red, or perhaps yellow. The goal of color balancing is to make the colors "right." We want white to actually look white and for gray to look gray since then in all likelihood the skin tones of the people in the photo will be corrected and look normal and pleasing to us as well.
COLOR AND THE COMPUTER WORLD
When computers render color, so that we can see color on a monitor or print it out on paper, they need to do so in a mathematical format. There are all sorts of complexities that enter into the display or printing of color but the standard that home computers rely upon most is going to be RGB color. With this system there are three primary colors, RED GREEN BLUE. Colors displayed on a monitor are a composite of these three, with each color have a value of RED, GREEN, BLUE between 1 and 255. Pure Black is 0-0-0. Pure white would be 255-255-255. Every other color, all 16,581,375 of them, are a composite of the three primary colors.
Whenever there is an equal mixture of RED, GREEN, and BLUE we get GRAY (or NEUTRAL) TONES
DIGITAL PHOTO COLOR CORRECTION
When we take a digital image, the camera makes it's best guess regarding the "Color Temperature" of the light source and the camera firmware adjusts the color accordingly. Sometimes the result can be very good, other times it is way off. It depends on the camera, the way you have it set, the degree to which the light is made up of mixed light sources, and a lot of other things. Fortunately we have wonderful software that can fix these problems for us and give us a great range of creative options as well.
Getting the proper color balance of an image is probably the most basic step in image editing. But there can be tremendous confusion among digital camera users about how to get a good result. Some software, such as Adobe Photoshop, appears absolutely alien to beginners. It takes a long time, some dedicated study, and lots of experience to really master Photoshop. But the most essential tasks, color balancing and tone control, are really pretty easy in Photoshop and most other image editing programs.
ENTER THE GRAY CARD
One way to "make a photo look right", is to put something in it that has a known value. The software can then correct the color of this one item, "pulling" everything else in the photo along with it, and making a very nice overall improvement. Let's say we have a photo that has a "Reddish Cast" to it, in other words the entire photo looks just a bit too red. To fix our problem we put something in the image that is neutral ( any color in which RED VALUE = GREEN VALUE = BLUE VALUE) and then let the software make sure that that object really is neutral. This will bringithe entire photo towards more realistic color.
To use a Gray Card, simply take a picture of it in the same lighting your subject is (or was) in. Use auto white balance and P (Program) mode for the picture. You can either use the resulting picture as the basis for a custom white balance setting, or you can use the Gray Card picture during post processing for a custom white balance.
To use the Gray Card as the basis for an in-camera custom white balance, make sure enough of the center of the frame is covered by the card in your picture to be properly read for white balance. Next, go through the menu to select that picture for your custom white balance. Finally, set the camera to the custom white balance setting.
If you are using the Gray Card picture as the basis for your post processing white balance setting, the card does not have to take up much of the frame and does not need to be exactly centered. Simply select a point on the card in the Gray Card picture from your post processing software to be the basis for the custom white balance setting for all pictures taken under the identical lighting conditions.