225. You are underway and pass by a lighthouse. Its light, which was white since you first sighted it, changes to red. What does this indicate?

A change from white to red light from a lighthouse indicates entry into a sector marking shoal water or other hazards. Lighthouses use sector lights to show safe and dangerous areas, with white typically indicating safer water and red or green indicating hazards. This color change signifies a shift in your bearing relative to the lighthouse, placing you within a charted danger area; therefore, a course correction is necessary.