Here's a Gray card, 18% gray and a larger gray card, showing the white side. I will photograph both cards and TRY to remember to remove them for the real shot! In Photoshop and many other programs, you can grab an " eye dropper" tool and hold it over the white part of the card, after the image is on the monitor and the 3 groups of numbers, should all be about the same, r-250, g-250, and b-250. If you get these numbers, then there is no color caste on the image and the capture is perfect. If the red is 10 or 20 points higher or lower in this case, then there is a cyan caste in the lower example, and you would have to add about 20 poitns of red to balance the color. The highest number for white is all three R,G,B, to be at 256 each. Most printers can't print any detail that high and I use the number of 250, as my limit to keep the whites from getting blown out!