Link: Konkeptoine

Simple and straightforward testing.
Based on the COne's tutorials.

Pile up shaders from bottom to top.


Simple plane with EI's standard Bricks3D.

Applied PitsD (bump only shader) over the Bricks. Reactive mode for Bump Strength is set to Desaturated Areas, so that mortar has more distinctive effect. 
Reactive modes are simple and easy to use. You just set the amount, and choose a reactive mode from popup menu.
With Desaturated Areas mode, shader effect is stronger over the less saturated areas. Suppose you set the amount to 1.8. Then, you can get 1.8 over the zero-saturation areas, while no effect over the 100% saturated areas.

Add DirtLayer over the PitsD. Desaturated Areas for reactive mode. Notice subtle effect on the bricks. Because bricks' saturation value is not 100%.
Note: If you set mortor's blue value to 100%, and set PitsD and DirtLayer's reactive modes to "Blue", bricks would have less effect. But I prefer Desaturated Areas for this example. Love this dirt.

Finally, add another DirtLayer with blend mode set to Multiply.  Reactive mode set to None. Instead, enable Moisture Mode to fade its effect from top to bottom. For one of two colors of DirtLayer, set alpha value to 0, in order to mask it.

Another test: Oak