Though some makers (Bachmann, Lionel, Mega-Steam, MTH, etc.) of fluids for smokers offer a selection of scents to simulate diesel exhaust or other scents I do not recall any that will offer color choices beyond the gray/white color.
From what I've read about efforts to make darker gray or black smoke, those employ oil based solutions or additives that deposit so much residue that this will eventually compromise the smoker's performance. I would avoid using these solutions since even though they can offer a good appearance they make a big mess requiring a greater clean-up effort, and risk staining the model and its surrounds.