In 7mm scale and larger, it is quite reasonable to use pigments to add texture without the look achieved being overdone. In the case of our Land Rover, pigment can be used to replicate accumulated mud on the wheels. A generous (for a change) amount of pigment is collected from the pot and deposited on the wheel hub, having supported the model on its side (hence the strange viewing angle).
When happy with the general arrangement of the pigment pile, a drop of white spirit is added to the hub. The pigment immediately absorbs the drop and a right mess ensues: