Light or barely visible smoke is your ideal - the blacker it is, the less efficiently the fuel (in this case coal) has been burned. There’s a number of factors which may result in the blacker smoke - yes sometimes it can be done for show, but can also be a result of how skilled or experienced the fireman is, the quality of the coal being used is a factor for sure, the loco itself may be a factor in some cases like 15 ‘Hastings’ at the KESR which is a smoky little engine at the best of times and of course how hard it’s being worked - gradients like on the Watercress Line require more power, thus more coal. So there’s no single answer as such for why it happens but light/ barely visible exhaust is a sign of a good team on the footplate who understand the fuel burn of that loco stand are working it well. So full credit to the driver and fireman in this video who worked Manston to perfection 🙂