I've attached a radio wiring diagram for a 2001 Audi A6 Avant. I'm sure its very similar to your 03, but sometimes the wire colors do change slightly. I'm not sure it will answer your questions about the wire colors, as it does not give the purpose of each wire, just the connections.
I really don't think you should need to figure out all the colors anyway. Metra makes a wiring harness adapter for your car. I've used this exact adapter on an S4 and it worked great. If you have the Bose system, you use all 4 RCA jacks. If you have the regular concert radio (only the rear speakers use an external amp), you use the two rear RCA's (green and purple) and you use the wires (white and gray pairs) for the fronts. There is a blue wire that is responsible for turning on the external amp. I always recommend using an adapter like this instead of tapping into or hacking off the factory connectors.
Metra also has the appropriate antenna adapter as well:
I hope this answers some of your questions.