I gather that British coal, which was mostly bituminous coal I think, would make a different color smoke that the then available American coal. I imagine it was all part of an effort by the North to lull Southerners into thinking that they were not American ships. Obi Wan voice: "These are not the American ships you are looking for."
Similarly, the Powhatan, the ship that Lincoln secretly diverted from the Fort Sumter fleet to instead aid the effort to reinforce Fort Pickens, had been intending to sail into Pensacola Bay flying "English colors" until Meigs stopped them.
Wow, you guys have solved the enigma! Lincoln was secretly working for the Rebel Alliance and on a mission to disrupt the evil forces of the Empire.
*SMH*