On Flightradar24, the color of the line (or trail) behind an aircraft represents its altitude at that specific point in the flight. As of 2026, the color-coded system uses a spectrum where different colors indicate different altitude levels. A light blue to teal line typically indicates that the aircraft is at a mid-range altitude, often between 5,000 and 10,000 meters (roughly 16,000 to 33,000 feet). As the plane climbs higher, the line will transition into a darker blue, purple, and eventually red for the highest altitudes (typically above 13,000 meters). Conversely, lower altitudes are shown in yellow and green, with white representing altitudes below 100 meters (such as takeoff and landing). If you see a black dotted line, this indicates that the aircraft is currently outside the real-time coverage area (often over an ocean or remote territory) and the system is estimating its position based on its last known trajectory and speed. This color-coding allows users to quickly visualize whether a plane is climbing, descending, or cruising at a stable altitude, providing a 3D-like perspective on the 2D map.