Avocado Crema

I'm a big fan of everything avocados and guacamole (I know, my millenial is showing, but love me some avocado toast!) To make this crema, I start with the same ingredients for guacamole and then add some sour cream or yogurt - then some sort of science happens that keeps the final product looking gorgeously green for days instead of hours. I made this for shredded beef tacos one night and ended up with some leftover crema - it was still the same pretty green the following evening when I ate it again for dinner! Unclear if it lasts longer than that because it was gone at that point...