The best way to keep guacamole green via thekitchn.com:

1. Make your guacamole . Put the guacamole in a bowl that has a tight-fitting lid. Pack the guacamole tightly in the bowl, pressing out any air bubbles. 
   
   
2. Dribble in some lukewarm water. I like to fill a measuring cup and pour it gently down the sides. Make sure the water covers the surface of the guacamole. I use about 1/2 inch of water. 
   
   
3. Put on the lid and refrigerate for up to 3 days. 
   
   
4. When ready to eat, take off the lid and and gently pour out the water. 
   
   
5. Stir up the guacamole to incorporate any extra moisture.