Washes but they have to be removable so it is either oil (can be tricky) or clay based such as Flory washes which can be daubed over the model, allowed to dry and then gently wiped off using a damp tissue :eek: and cotton buds leaving the wash in the grooves. If it goes wrong, more water (wet cloth and brush) and it can all be washed off. Once you have it how you like, spray a varnish, I wouldn't brush on the varnish as it will lift the clay out of the grooves, and it will become permanent.
A similar effect can be achieved with pigment and water but it won't be as stable and would be more powdery as it drives in the grooves.