Great for adding
impurifications to your textures is the Variate function.
This function manipulates the texture's HSV
channels with a noise texture. While modifying the brightness
channel produces dirty textures, changing the saturation channel
results in a worn-out effect.
Regulate the variation strength with the Amplify
Transformation of the variation noise is
used to increase the number of effects that can be achieved with
the function. Just have a look at the Spots and Granitize