I am using PS CS2.

I am attempting to colour correct a digital image.

Suppose you place a grey card in a position within the image you are taking with a digital camera. When you display this digital image in CS2 and sample the colour of the grey card you find that the R G and B are not equal as you could expect but rather only close to equal. As an example they may show:

R = 170 G = 168 and B = 172.

I will pick, let us say, the G = 168 and then attempt to modify for the whole image the R and the B also to 168 so that when I sample the grey card it now reads the same values for the three colours. I guess my question is how do you modify the colour of my whole image by a number, plus or minus, rather than the percentage figure that appears when you use IMAGE/ADJUSTMENTS.