My Cuban friend says to always put the avocado stone in with the prepared guacamole. It seems to work, but guacamole never really sits around that long in my house...
All guacamole will turn brown. One thing to keep the avacado green longer is to add lemon juice to the avacado before adding other ingredients. Always serve fresh
lemon or lime juice to act as an antioxidant to prevent the iron in the avocado from oxidizing. Iron is the reason why avocados turns brown in the first place.