I would absolutly expect the coolant to bubble unless you were to allow the engine to completely cool down befroe removing the cap.
If there was a problem with the engine overheating due to any type of combustion entering the cooling system, you would quickly overheat.
Please ask me any questions that you have and I will be happy to answer them...Thanks, Josh
As far as the coolant condition, If the rest of the vehicle appears clean and well maintained, it doesn't surprise me that the coolant would be as well. A well maintained engine will keep fairly clean coolant.