It's usually the inside rim lip closest to your nose that darkens as it's inevitable due to the way the pipe hangs from our mouths. The only way to avoid it is to keep the pipe perfectly horizontal, and that's just silly and would look even sillier. No matter what you do, anything coming in contact with direct smoke is going to darken over time, including your teeth and even the walls. It's unavoidable but regular maintenance is all that is required to keep it from forming any buildup. But staining from smoke does not bother me. The lava buildup does though and is easily avoidable with a bit of spit and a wipedown after every smoke. An unforgivable sin, however, is burning your rims with a lighter, and I too wet the rim a bit before lighting to avoid that.
As far as cobs go, of course I don't worry as much about them. But they are the easiest to maintain. The lacquer used on MM cobs is not alcohol soluble, so wiping down your cob rims with an alcohol soaked rag or Q-tip keeps them looking nice. You can NEVER do that to a briar or you'll ruin the finish. I've had very good success with Murphy's Oil Soap on briars, but prolonged contact will also cause issues. Be quick, get off what you can and rinse it under the sink, then dry with a towel. I've never had any problems drenching my pipes in warm water inside and out briefly.
As far as cobs go, of course I don't worry as much about them. But they are the easiest to maintain. The lacquer used on MM cobs is not alcohol soluble, so wiping down your cob rims with an alcohol soaked rag or Q-tip keeps them looking nice. You can NEVER do that to a briar or you'll ruin the finish. I've had very good success with Murphy's Oil Soap on briars, but prolonged contact will also cause issues. Be quick, get off what you can and rinse it under the sink, then dry with a towel. I've never had any problems drenching my pipes in warm water inside and out briefly.