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