Here's a cheap and quick way to colour code your keys: Cover 'em with balloons. They're easier to get on than the rubber covers you get from the hardware store, and you have lots of colour choices at your disposal.
The Zakka Life blog posts the simple directions for using the top of a small water balloon as a key cover. This clever hack is pretty self-explanatory, though. The next time you find yourself at a party, be on the lookout for broken balloons on the ground, so you can reuse them as your key covers.
Life Hack: Balloon Key Cover [Zakka Life]