The beacon for a land-based civil airport is an alternating white and green light.

There are several different light combinations for airports and helipads.  These are:

White-Green: Lighted land airport
White-White-Green: Military land airport
White-Yellow: Lighted water airport or seabase
Green-Yellow-White: Lighted heliport

Reference(s):

AIM 2018 2-1-10
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 14-16

Other Airport and Helicopter Topics