Most control - use an image texture, and layer on top of that a "dirt map", basically a texture that has been desaturated and contrasted up to be just basic values of gray.

Trickier - if you go with a Bryce procedural DTE .mat...I normally will take a bead over from my Diffusion (in the Value row) into an open channel, and try to choose a texture that will dirty it up. But, like I said, trickier.

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