The simplest approach is to add a Displacement Shader (that's actually what the shader/node is called, though "Displacement Shader" is also a *class* or category of shaders). Connect it in to the main shader network, if you are adding it in the Node Network and not using the Node List buttons on the left (the latter is easier, it auto-connects it for you). Then connect the Painted Shader output to the Function input of the Displacement Shader (input on the right). Change the Displacement Multiplier to adjust the effect. It sounds like you want negative, so try -100 as an initial test and adjust to your preference.
- Oshyan