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Generally in the context of image processing, using a "well behaved" colorspace such as sRGB or Adobe98 or ProPhoto, a pixel is neutral if R=G=B. For example, R=128 G=128 B=128 is a middle shade of gray.
What I meant, from my readings, it instructs to make an image neutral by clicking on parts of it with the white balance tool.
Clicking on the image with the white balance tool forces the three channels to converge so that the pixels under the mouse pointer will end up with R=G=B, with a luminance approximately equal to the original image. How well this creates a "neutral" image depends upon several things, including whether or not the object selected is truly neutral, differences in lighting throughtout the scene, etc.
The idea is if you can find a white shirt or similar object that isn't blown out and you click on it with the while balance tool, then the whole image should look more natural to the eye, i.e. little or no color tint in areas you know to be some shade of gray, including black and white.
Personally, I almost never get satisfactory results using this technique. - very few things in nature are truly neutral (maybe I need to use more bleach when I do my whities). Not to mention that most light casts a color on even neutral objects, so perfect neutral adjustment often looks unnatural.
That said, one thing to consider is to plan for it - shoot a white or grey card in the scene at some point and use that as neutral... (as Richard S pointed out, it doesn't matter whether the point is dark or light, as long as its got the same amount of r, g, & b in it.
…from my readings, it instructs to make an image neutral by clicking on parts of it with the white balance tool.
CORRECTION: You want to make sure the neutral colors in your image are truly neutral (not the whole image).
As explained to you by the posters above, "true neutral" can be any shade of true gray, from pure white (255, 255, 255) through pure black (0, 0, 0). That's just the definition of neutral so you understand what we are talking about. Obviously, the numbers of any given pixel in your image tell you absolutely nothing about what should be neutral. That's when your brain should kick in.
You can use the White Balance tool only if you have am object in it that is supposed to be a true neutral, otherwise the tool is of no use to you. If you have an image of a yellow house with a red tile roof and surrounded by green grass and blue skies, the White Balance tool is of no use there.
Also, you can only define a single point with the tool, if you go around clicking on "on parts of it ", as you type, you're just wasting your time. Each click will undo what the previous click did.
Wo Tai Lao Le
What do you do if you have "an image of a yellow house with a red tile roof and surrounded by green grass and blue skies." How do you then use the White Balance tool? Do you instead just manually adjust the temperature and tint?
The white balance tool is utterly useless in that situation.
Your brain and your eye kick in.
There are other ways of adjusting colors other than the color sliders. But here we get into the area of teaching you how to use the application step by step, and that far exceeds the scope of what can be accomplished within the scope of the forums.
Get Jeff Schewe's book:
Wo Tai Lao Le
The reality is that there's usually something in the image that should be just about neutral, and which you can click on to make the color balance of the entire image look good.
Also note that there is a similar "White Balance Tool" dropper in the Camera Raw conversion dialog, which can be very handy.
Practically speaking, just click on different things that you see look colored that shouldn't be, or that have a tint you don't want, until you like what you see.
And don't forget that the dropper tool can average a sample of more than one pixel. You can set this by right-clicking in the image with the dropper cursor active. Try not to forget it set to something more than "point sample", because that can be confusing later.