Neutralize the Neutrals with "Match Color"
Photoshop Tips

There are some things in this world that we expect to be neutral. 18% neutral graycards, for one, but if you're out shooting these you probably don't need to be reading this article. Other objects that we expect to be neutral, such as bare cement, white picket fences, unpainted aluminum, black mercedes, gray studio backgrounds, are often close to neutral but not quite. Yet it is still bothersome to see these objects in print with a decided color cast as we have come to expect neutrality here and use these objects to reference the color of OTHER objects. It is very useful to have a technique to neutralize the image based on the actual color of these key objects and not just what we THINK their color is when we are editing on the computer. And it is virtually impossible to do this by eye, looking at the computer screen.
Our perception of color is substantially influenced by surrounding colors, by what we looked at just before, by expectation, by mood, time of day and what we ate for breakfast. What we accept on the computer screen is often not at all acceptable in print form. Looking at a printed photograph we key off of all the white and neutral objects around us, including the white paper base and other white and neutral objects in the room, to establish our mental reference for color balance. Any hint of color cast is always easier to spot in a print, for whatever reason. Perhaps, when looking at a print we are more likely to use environmental neutral and white objects around us for our mental white-balance. Working on the computer, our mental white balance is coming exclusively from the screen, particularly from the brightest white areas. For those reasons, this technique useful as a way to ensure that the neutral areas actually ARE neutral. Basically it is a way to remove color cast based on the color of an arbitrary neutral area of the print. The actual method is easier than this description.

HERE'S HOW: Using the Wand or Marquee or Lasso, select part of an area that you want to be neutral. With the shift key down, add a couple other sample areas to the selection to be neutral. Go to Image/Adjustments/Match Color. Check neutralize and also check Ignore Selection. The entire image should now be color shifted so that your selected areas are neutral. In other words, the software came up with whatever global color change was required in order to neutralize your selected areas. Use the fader to vary the degree of color change

Scroll down to see all of the images to the right - which gray patch is neutral? A couple are pretty far from neutral - one only is truly neutral. (answer below)
The 1st three are all the same and are somewhat blue (R83 G86 B94)
The 4th is somewhat red (R138 G131 B131)
The 5th is somewhat blue (R83 G86 B94)
The 6th is a bit magenta ((R135 G132 B136) and, to answer the question,
the last is neutral (RGB=52. )