Short answers, pulled from the story.
White is the lightest possible color because a perfectly white surface diffusely reflects and scatters all visible light that strikes it without absorbing any, irrespective of the light's wavelength or spectral distribution. Since it keeps none of the incident light, no color can be lighter.
White on a television or computer screen is created by mixing the primary colors of light, red, green, and blue, at full intensity, a process called additive mixing. White light can also come from the sun, stars, fluorescent lamps, white LEDs, and incandescent bulbs.
Isaac Newton demonstrated in 1666 that white light is composed of multiple colors by passing it through a prism to break it into components, then using a second prism to reassemble them. Before Newton, most scientists believed white was the fundamental color of light.
The pope has worn white regularly since 1566, when Pope Pius V, a member of the Dominican Order, began the practice, with white symbolizing purity and sacrifice. White is also the color of the Dominican Order, and earlier popes wore white only occasionally and usually wore red.
In China, Korea, and other Asian countries, white, more precisely the whitish color of undyed linen, is the color of mourning and funerals. In traditional China, undyed linen clothing is worn at funerals and small sacks of quicklime, one for each year of the deceased's life, are placed around the body.
A white dwarf is a stellar remnant composed mostly of electron-degenerate matter, with a mass comparable to the Sun's and a volume comparable to the Earth's. Its faint luminosity comes from stored thermal energy, and since it has no energy source, it gradually cools and reddens over time.
A white flag represents surrender or a request for a truce and is believed to have originated in the 15th century during the Hundred Years' War between France and England. It was officially recognized as a request to cease hostilities by the Geneva Convention of 1949.