Getting back to topic, I've not heard of brown smoke. Searching for the topic gives mixed results, but generally centres around the following possibilities:
* poor quality fuel
* gunk in the CAT being burnt off
* rust in the exhaust somewhere being blown out
* mice being cooked inside the exhaust
I don't subscribe to the last two at all. Rust wouldn't last long and would only be evident if the vehicle had been stationary for several months beforehand.
I'm going to go with fuel and CAT, maybe a combination of both. There's a possibility of getting dirt or fuel tank rust picked up by the bowser pump at the servo, or they're using a bio blend maybe?