If I need to store guac, a layer of lime juice and plastic wrap without air-avo contact keeps it green and good. Can't say I've tried keeping half an avocado but I suspect the same technique could work if the acidic flavor isn't a nuisance (there might be alternatives to lime). Maybe a spray bottle could apply it.

I just stick an avocado half in a plastic container in the fridge. Stays green enough for days. If it gets brown, it's a thin layer that doesn't affect the flavor anyway and you can remove it without losing too much avocado if you don't like it.

Here's a secret hack to bring back the flavor of old avocado in the fridge for a few days: a half teaspoon of soy sauce and a few drops of hot sauce. You can thank me later.