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In the period of decay leading up to flashover, one of the significant observable changes in the fire environment is the alteration in smoke color. As the fire consumes available fuel and oxygen, the combustion process becomes less efficient. This inefficiency often results in a change in the color of smoke, typically transitioning from a darker, more oppressive black to a lighter gray or potentially a yellowish hue. This shift indicates the fire is burning hotter and consuming more materials, which can precede an imminent flashover.

In contrast, rapid flames are typically associated with a more active fire, not the decaying phase. A temperature decrease would indicate a reduction in the fire's intensity, which is opposite to what occurs in the decay stage. Similarly, visibility improvement would not be expected, as smoke tends to increase during this time, leading to diminished visibility. Hence, observing changes in smoke color serves as a crucial indicator of the fire's condition and its potential progression towards flashover.