White has significance in several cultures, many of which attach similar meanings to the color. The most popular associations include purity or cleanliness. We see this in the cultural tradition of America and other countries of a white wedding gown, symbolizing the purity of a virgin bride. Similarly, in Judeo-Christian tradition, characters thought of as pure are often represented in white robes, i.e. saints, angels, Jesus, etc.
These visual representations often lead to another association of the color, which is innocence. Again, this has religious significance, esp. in Judeo-Christian societies, but within the greater community white often involves an absence of evil, and therefore an abundance of innocence.
Finally, white can also be a symbol of death. This is the closest association in Fahrenheit 451. The idea of innocence/purity plays into this, as in death, one is cleansed of sins. See also Whitman's "A noiseless, patient spider."
For me, the color represents, as I really think it does/did to Bradbury as well, the empty page a writer sits before each day he/she works. It is full of so many possibilities, and Bradbury, freed by a circus magician who gave him 'everlasting life' by touching him with what he called 'lightning' -some electrical shock, mild, was by a young age ready to fill reams of white paper with many many words. Fahrenheit 451 shows the potential of paper: in its ashes after it burns, in the way it can sear a story into memory to be told and retold- that's how I see the color 'white' in this particular book. Good question and glad you asked it for me to think about, once again. Eng. Lit. degree, made living as writer, editor and tutor. Retired. Name here is Troubador. Keep at it, Windfaller.