What is white? What’s supposed to be white? Should white be white?
In the print photo world, white is the paper base. No color has been laid onto it. For B&W there is no emulsion buildup, just the color of the underlying paper. Zone 10. No texture.
In the RGB digital world it’s 255-255-255. Equal parts of R, G, and B at the highest measured level. Any equal combo of RGB is called gray. 0-0-0 being the darkest gray (AKS “black”) and 255-255-255 being the brightest.
So maybe we should call auto white balance, auto gray balance.
White balance is a matter of taste, really. Not many light sources in the real world are actually white. They all have some color tinge and our cameras often exaggerate the tinge. Regular light bulbs give us an orange cast, LEDs are bluish. And fluorescents are a sickly yellow with a hint of green.
So how do we know what is supposed to be white. I don’t really know.
Maybe we have a built-in auto white balance in our brains that balance out the colors to make sense to us. We see the coloring cast from different light sources but make mental adjustments.
The camera has to do this electronically. How does it do it? I don’t know that either but it works pretty well most of the time except for when we are confronted with mixed sources of light. My camera doesn’t have the mental capacity to adjust different parts of a scene, differently. Fortunately, I can do that later on my computer (that’s why I shoot RAW).
Whatever we call white balance usually means we are just making our photos look like we want them to look.