Are you referring to a color correcting "pen" using white or a neutral gray as a reference? I have CS5 but I am sure CS6 is the same.
With the file open, go to the color tab on the right. Click on levels and click on the eye dropper in the middle. A box will come up that says "Sample in image to set gray point". Click on a white or NEUTRAL GRAY area to correct the color
One thing to remember is when you do this the info will be in another layer so you will have to go back to "background" to do anything else with the image.