Both the flame itself and the smoke associated with a fire offer important clues as to the nature of the fire. Different substances, whether wood, plastic, gasoline, kerosene, etc., burn differently, and each produces a distinctive flame and smoke. While the obvious first priority of a fire company responding to an alarm is to extinguish the flames and extract any people (and animals) found within the burning structure, attention is also paid to the physical characteristics of the fire. Just as household fire extinguishers are rated for different types of fire (e.g., electrical, wood, grease and oil, etc.), so are structural fires to which fire fighters respond diverse in terms of cause and effect.
Materials used in the construction of buildings, as well as substances found in nature, all have some chemical composition that distinguishes them from each other when burning. Wood, for example, contains carbon and oxygen that burn blue, as well as sodium, which burns orange. Fires also vary in temperature, with colors emitted reflected in the spectrum—blue-violet flames the hottest and red on the cooler side of the spectrum. Because different materials, comprised of different chemical compositions, burn differently—some hotter than others, some burning out quicker than others, etc.—there is abundant evidence to be identified in what can be observed while extinguishing a fire.
When responding to a fire, fire fighters try to locate the source, which is both vital to their ability to extinguish it and to identify the cause of the fire. Skilled arsonists will usually attempt to obscure the physical evidence they know will exist, but the presence of unusual fuels in a given location as well any tools (e.g., matches or electrical wiring used to start a fire, especially a fire the initiation of which is deliberately timed to begin well after the arsonist has departed the location) can often, but not always, be identified and provide clues for a subsequent investigation.
The National Criminal Justice Reference System provides an abstract, a link to which is below, that summarizes the association of smoke color and fuel. The details in this paragraph are consistent with those found in other sources (also linked below):
Most fires involve carbonaceous fuels such as wood, paper, plastics, petroleum, or textiles. When these fuels do not burn completely because of a deficiency of oxygen, the conversion of carbon into carbon dioxide and water is impeded; and free carbon, or soot, appears as smoke. Some fuels, such as alcohols and cellulose (cotton or paper, for example), contain oxygen and tend to burn cleanly when air diffuses into the flame. Insufficient oxygen can also lead to a yellow flame because unconverted carbon particles glow yellow hot. In addition, many common materials contain some sodium or other elements that give yellow or other colors to flames. Whether a flame is light yellow, orange-yellow, or reddish depends on the temperature of the flame. The hotter the flame, the lighter the color. White or light gray smoke is usually associated with paper, straw, leaves, or wood. It is formed of pyrolysis products (gasses, liquids, and tars) that condense to form a fog of tiny droplets that bypass the flame. Other sources of white smoke include burning phosphorus, magnesium, and some other metals, but fires containing these elements in sufficient quantity are rare. Most fires will produce a mixture of black, gray, and nearly white smoke because of the variety of fuels and the variability of air supply.
Smoke is, as noted, an important clue as to the origins and stage of a fire. The longer the fire has burned, however, the more difficult it is to identify the primary materials that mark the fire’s origins. This is because the longer the fire burns, the more it spreads and the more diverse the items with which it comes into contact. Additionally, the process of extinguishing a fire can contaminate physical evidence, as chemical foams used by fire fighters can obscure chemical residues used in igniting the fire. Black smoke is usually associated with gasoline; brown smoke with wood; white often suggests the early stage of a fire which has not yet begun to consume wood, plastics, etc.; grey indicates that the temperature of the fire is dropping because the fire is being extinguished, perhaps due to a lack of oxygen but more likely to the efforts of the fire response team.