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The 18% Gray Card has several uses. It can be used to determine the correct exposure (hence the 18% bit) and can also be used in a couple of different ways to help set up the correct color balance for the shot

Exposure

Taking a light reading from an 18% gray card guarantees you a correct exposure, provided that your main subject is lit at the same level. Get your subject to hold the card and zoom in so that the card fills the frame. To get the best result, you need to set the exposure manually.

Color Balance

One way to get the color balance set correctly is to take a picture of the card, making sure that you fill the frame as much as possible with the card, and then store the

Keep the picture you shoot and you can use it in a different way in the computer. If you shoot in RAW format (and if not why not?), you can simply click on the gray panel with your dropper in Adobe Camera Raw. This will change the picture to make the panel neutral gray and all the other colors will change in relation to it.

If you are not using RAW there are still ways of changing the color in Photoshop. These vary according to which version you have, but one way to do it is to use the ‘color balance’ window in conjunction with the ‘info’ window. Hold the cursor inside the gray square and just keep tweaking the color balance until the info window tells you that all the color values are equal.

The other side of this gray card is in fact white, which makes it even more suitable for color balance adjustments. It seems that the color balancing works better if you work on lighter colors. In a lighting situation like the one above, where there are a lot of reflected colors around, especially the green from the trees, you need all the help you can get.

One word of warning though, these processes, although a great help, are not completely infallible so let your eyes be the final judge as to whether the color is ‘correct’. In fact I would go so far as to say that there is no such thing as ‘correct’ color, we get as near as we can but the final judgment is always subjective. If you like it, then it’s right.

Once you start looking at color more closely you will drive yourself crazy trying to get it just right. All printing processes and all monitors produce slightly different colors. You will see different colors in the picture above than me. We can only hope to get as near as we can. [via]

The zone system begins with a zone chart. Zone 1 is pure black; zone 9 is pure white; zone 5 can be described subjectively as middle gray. The zone five, the point if you use grey card.

see the video below to know more about gray card