What color indicates that the plane is on the glide path in a tri-color VASI?

The indication that a plane is on the glide path in a tri-color Visual Approach Slope Indicator (VASI) system is represented by the green color. In the context of VASI, the lighting system consists of three colors: red, green, and white.

When the plane is properly aligned and at the correct altitude on the glide path, the indicator will show a green light, which signals to the pilot that they are on the ideal descent path toward the runway. This configuration provides visual guidance, making it easier for pilots to maintain the correct approach angle, facilitating a safe landing.

If the light were red, it indicates that the aircraft is below the glide path, while white would be a signal that the aircraft is above the glide path. Amber is not typically used in standard tri-color VASI systems. Hence, green is the correct answer to indicate being on the glide path.