This utilizes the principle of "underlying transparency",
which can be seen on tutorials elsewhere on the 'Net.
Take your initial material, say it's bronze metal. You want to
add variation, age etc to it.
Decide on the material that will alter it, maybe a green landscape
to make it look verdigrised with age.
Ok, edit the material, go to the TRANSPARENCY tab, move slider
to 100%. click variable transparency, and UNDERLYING MATERIAL. Chose
Now, how you mix them can vary, but usually EDITING the function
and adding a fractal, tweaking it to taste, will provide a randomized
mix, very good for dirt/age etc. Note you can also make image MASKS
for precision, like an archway with a different type of brick in
the arch, driven by a mask!