Factual error: Right after Walter has the nightmare about killing his wife, he wakes up and turns around to see the alarm clock show 11:12. The clock makes a flipping sound when it switches to the next minute, just like those alarm clocks from the 1980s that have those little numbered plastic cards inside. However, it's a digital alarm clock and they do not make such sounds.
Continuity mistake: When Walter is imagining himself killing his wife, the bed sheets change from silver to red momentarily after he wakes up.
Factual error: Walter states that Ted Bundy died on January 23. He died on the 24th of January, 1989.
Factual error: Walter reads from his book he wrote in 1991. He mentions the Oklahoma city bombing. That didn't happen until 1995.
Answer: A guess: it matched the colour of the walls that she had chosen (red). It's also shown sitting on the shelf, facing outwards, making it more obvious.