Fig. D

and we obtain the output of Figure D, it gives the idea of a dirtier surface isn't it?maybe there's not enough color variation, so clone the previous FBM layer, change size or position if you want and assign another color..

Fig. E

Figure E is a render with another brownish fbm layer.They're also great to give color variation to monochromatic textures. Notice how we can rather clearly see the details of the metal plate texture we applied in the beginning! remember that the dirt also affect specular, glossnes, and bump channels so you should copy them on relative channels and play with values expecially if you're dealing with metal surfaces. This is just color channel and layers opacity are boosted for clarity, you should have a less evident effect depending on the kind of surface, anyway play with settings until you're happy!