Generally speaking color cards are used to acheive correct white balance (i.e. to correct any color cast from the light under which the photo was taken.) If your card contains an 18% gray area, you can use the Curves tool to select that area of the card (in your photo) and correct your color.
This is how:
It's worth noting — if you are processing a ton of photos, you may want to explore setting your camera to capture your photos in Raw mode (if it has this function) and opening the raw files in Photoshop. This will allow you a more accurate white balance through the Photoshop Camera Raw interface than jpegs and will also allow you to apply the white balancing to multiple images at the same time (only do this if the photos were taken under the exact same lighting condition).
Hope that helps,