I guess it's worth mentioning... some scenes have no pure white in them ( like a shot of some forest or whatever ). White balance is matching the light source (measured in Kelvin degrees ) to the camera WB setting. It gets a little more complicated in pro work on film sets, because the camera will shoot color chart (and tone scale) with dedicated light source to match the camera ( let's say you are shooting indoors with tunsten light ( 32K ) you would shoot the chart with a 32k light only. THEN, to get the mood you want, you may put color gels on the lights of the actual set ( for the scene and skin tones, etc. ) to make it warmer or cooler etc. In that case the editor matches to the chart shot , and NOT the clips. There are color temp meters and color correction gels available to control this stuff. So, for most people, not working on pro sets, it's really a subjective choice how you want the skin tones to look, the green of trees, and your whites ( if there are any ). That is why the eyedropper is not that reliable by itself as the only tool to try and get your proper look.
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