No-one thought about the smoke trails...
Not really relevant, The amount of smoke would be dependent on the aircraft's power settings and the smoke visibility would be dependent on whatever overcast was behind it, I think you would find that the main purpose of the trial was to see if the Brown aircraft was more 'visible' than the White one. Seeing that it wasn't and they were picked up at same instant, the trial was basically successful. As Nimbev has correctly stated the main purpose of the hemp scheme was to camouflage the aircraft while it was sitting on the ground.
The ASW scheme has some documented history to it, Operational research in 1941/2 found that an all white scheme gave the shortest detection range for visual pick up of a Coastal Command aircraft by the crew of a U-boat in daylight (compared with the original day fighter / bomber scheme used) . Thus most of the fleet of Wellingtons, Sunderlands and Liberators were painted white, with Extra Dark Sea Grey upper surfaces to help camouflage from above while over the sea from fighters (when operating over the Bay of Biscay or north sea for example). Advent of the snorkel and the later Nuclear Submarine, lead to the all over Extra Dark Sea Grey scheme being used as camouflage from observation from above, however most likely due to solar heating effect reduction in places like the Med and Far East, the top of the fuselages were white. Nimrod MR 1, White / Light Aircraft Grey, Aircraft did high level transit, solar heating effect reduction were paramount, plus type was based on Malta. Hemp, Ground Camouflage. Camouflage Grey at phase out. Best compromise for low / medium altitude.