Instead take a look at figure B: it is a green FBM noise applied to a white surface with default lightwave settings.

The first thing we notice is that we dont have a green color covering everything but some nice random splotch all around the surface, FBM noise yet lets us notice the white behind it.

Figure B

lets apply this and dirty a metal surface for example

Fig. C

figure C shows a metal image map applied on the previous plane,it has some nice detail but we want to dirt more so we apply a layer of black FBM noise over it