let me add a little bit in here....
The white steam is effectively the same effect as when you go outside on a cold day and see your breath. What you are seeing is condensed water vapor that turns to steam when it hits the atmosphere. You get the same effect when you open the cylinder cocks starting off. Water tends to find it's way there and it doesn't compress very well. Opening the cylinder cocks lets any accumulated water out of the cylinders. If it's cold/damp, you see steam. If it's hot/dry, you likely won't see any steam.
As far as the gray/black smoke, it has a lot to do with the firing of the engine and--if coal burning--the quality of the coal. With a good fireman and good coal, you may not see much smoke at all. I'm not sure what kind of coal the TVRM crews were using with Southern 630 this summer, but it was very difficult to get any smoke at all from that thing. Good coal and good firing won't make much smoke.
Sometimes though, you have to resort to overfiring the engine for whatever reason. Basically, that means adding more fuel than you would normally use under the circumstances. It may be poor coal that doesn't have good heating capacity and you have to overfire to keep pressure up. Sometimes the overfiring is done intentionally to "make a show" for the fans. Either way, what doesn't get burned goes straight up the stack as smoke. Basically, for oil burners, the same rules apply.
Depending on weather conditions and such, you will often get both steam and smoke at the same time.
steam (white) + smoke (black) --> varying shades of gray.
The other possibility on oil burners is sanding the flues. Coal is abrasive enough that you don't get soot on the flues (which in turn makes it harder to keep pressure). With an oil burner, sand is used for the abrasive action. Since you are clearing out soot, you get smoke....very, VERY black smoke, and lots of it. Obviously, this can be done for runbys and such also.
Hope that helps...
Kevin
