Guacamole is best when eaten fresh. When placed in the fridge it quickly turns brown, and while the acidity from the lime will help to slow this down it will still happen sooner rather than later. But! Even if it turns a bit brown, you can still eat it. It's still delicious. It' just not as pretty. If you do have some leftovers, cover and store in the fridge and you can eat it the next day as well (even if it's a little brown).