The upper surfaces of a steam locomotive receive copious amounts of grot from the exhaust and, sometimes, the surrounding environment. This locomotive is intended for use in a quarry, so there will be some dust added to represent that. Later!
To begin with, the soot from the chimney will be represented by a predominantly black mix of Railmatch Frame Dirt and Weathered Black. Application is from my trusty Iwata Eclipse SBS, with successive light coats aimed roughly at the top of the smokebox, saddle tank and cab roof. Why roughly? Because I don't want a sharp edge to the colour, but a fading away as it gets lower down the sides.