What color are the threshold lights used in aviation?

Threshold lights are used in aviation to define the beginning of the runway, providing critical information to pilots during landing and takeoff. These lights are specifically colored green. The reason for this color choice is to signify to pilots that they are approaching the usable part of the runway. Green lights are universally understood in aviation as an indicator of a safe area to land.

In contrast, the other colors listed are used for different purposes. For instance, red lights are often employed to indicate the end of the runway or areas where landing is not permitted. Yellow lights can be used in other contexts, such as runway edge lights to mark taxiways or hold positions. White lights are typically associated with runway edge lights rather than the threshold itself but do not convey the same meaning as green lights in indicating the start of the runway. Therefore, green is the correct answer as it is the universally recognized color for threshold lights, signaling to pilots that they are entering the runway.