I hate it when my beautiful avocado stuff starts to "turn" and gets dull and brown.   Lula's for Lunch...and More! Catering  has to make a pretty large volume upon occasion and sometimes it has to sit in the fridge for a couple of days - and there's just not enough acid in the recipe to keep it in the "Kermit" family, SO.....I cover the surface with a thin film of WATER.  The guac is dense enough (and fatty enough!)  that it won't absorb the water - so when I'm  ready to use it I just tilt the container and pour the water off and voila!  it's just as vibrant as when I made it!