I don't know if anybody's tried this before, but here goes. I got a couple of metal textures, and a bumpmap. Now, normal Anim8or procedure is to have the same texture in the diffuse and ambient channels, but I tried something else.
I put the bump map into the ambient channel and set it to -1. In the "Textures" section of the material editor, I set the ambient to "Lighten". Next I press the little "divorce" button that unhooks the ambient from the diffuse, and set the ambient color to black.
I put the texture map into the diffuse channel and set it to 2.
Behold - the bumpmap acts as a dirt or grunge map.
I've experimented with putting the bumpmap into the specular channel, and/or setting the diffuse channel to "Darken". You get subtly different effects.
Note - in Scene mode, increasing the ambient light in the scene makes it DARKER when you use negative ambient materials. Also, it can flip the colors into negative as well, so it works best with greyscale maps.
There's two renders here, one grunged, the other with the "vanilla" settings that you get with the material editor.
