The black key is the "Master Key". The grey key is the "Valet Key". The only difference is the valet key won't open the trunk or glove box. As long as you own at least one functional black master key, you can add new keys.
You don't need the code numbers, unless your own key is too worn to use as a template to cut a new one. If your key is worn out (or snapped in half), the dealer can use the code numbers to cut a new one from scratch. However, you would still have to buy a blank key first and then program it afterwards. The dealer will charge several hundred dollars for this service.
Instead, you can buy a blank replacement key cheaply, ask any hardware store to cut it to match yours, and then program the Prius to recognize the new key's chip all by yourself.
I did this myself a few years ago. I bought my replacement from Coastal (http://www.coastaletech.com/carkeys.htm
), but I see they don't carry them anymore. This poster (http://priuschat.com/forums/gen-ii-priu ... ource.html
) says he got one from eBay for $13.98, so I'm sure you can find a new source by Googling "prius replacement key". For example http://www.autotransponder.com/1998-200 ... c_chip.php
, but I have no experience with this particular seller.
The programming instructions aren't hard, but you might have to try it a few times to get the timing correct. It involves opening/closing the driver's door a few times, turning the ignition off/on a few times, etc. Here's a tip that helped me: when the instructions say "close the door" or "open the door", just hold or release the micro switch in the door frame that the car uses to sense when the door has been closed or opened. (If your key doesn't come with programming instructions, I know they are in the repair manual, and I think these look like the correct instructions for a 2001: http://www.coastaletech.com/keys.htm
Note: This information only applies to a 1998-2003 Prius. I believe the 2004+ models have a completely different ignition system.
Douglas (2002 Silver, Wisconsin)