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 value.

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 presets.