Final answer:
Brown or dark smoke signals an imminent collapse in structures with lightweight engineered wood products, due to compromised structural integrity.
Step-by-step explanation:
When observing brown or dark smoke coming from lightweight engineered wood products, it indicates that collapse is imminent. Engineered wood products, due to their construction, can be less stable under fire conditions compared to traditional solid wood beams. As such, firefighters must be extremely cautious as the indication of brown or dark smoke can mean that the structural integrity of the building is compromised, increasing the risk of collapse during firefighting operations.