The brain does a little trick called chromatic adaptation. It finds the brightest object around, and if it is not real saturated, it says, "That be white."
Have you ever approached a house and noticed that the incandescent lights inside looked yellow? Then you go inside and look at a white sheet of paper.* It will appear yellow only for a fraction of a second before the brains make it "white."
Sunlight is intrinsically greenish yellow. Yes, sunlight contains all the wavelengths of the rainbow, but not in equal proportion. Our good old brains sort it all out and invent white almost regardless of the illuminant. Brains are good at it. Chromatic adaptation for digital cameras is tricky. That's what those "daytime" and "cloudy" settings and all that "white-point" business is about.
* A "white sheet of paper" can be defined as a sheet that reflects and scatters almost all of the radiant energy in the visible spectrum that hits it, without altering the energy.
"Fully half the quotations found on the internet are either mis-attributed, or outright fabrications." - Abraham Lincoln
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