One has many keys on a keyring. The keys are more-or-less identical.
How can one modify a key to make it stand out from the others when rifling through them with one's fingers? It is dark and any visual distinctions cannot be perceived.
Most of the answers contain visual distinctions (nail polish, permanent markers, colored key caps, stickers, spray paint, etc.). Drilling holes and filing notches into the keys were recommended, but this is a hassle and requires special tools.