More work with pigments now, to add to the random dirty bits that have been created with Black Smoke. I normally use MIG Productions Industrial City Dirt or Rubbel Dust (that's what it says on the jar) for lighter stains, like ash and sand, and Dark Mud and Track Brown for rusty bits. In this case it will be Rubbel Dust (ash and sand) and Dark Mud (rust).

The lighter colour represents ash staining under the smokebox door and sand staining around the sandbox fillers, and the darker colour is applied to seams in the tender and then brushed outwards from there in random directions. The aim in both cases is to just hint at discolouration rather than have large deposits of different colours, and this is effected by using very small quantities (heard that before?) and brushing them out until the effect is achieved.