Undercoat of whatever the base material was, in this case aluminium. Layer of hairspray* on top, leave to dry. Next coat(s) the camouflage colours, again let dry. Then with an old brush cut down to maybe 1/5 of original bristle length, wet the top coat with the appropriate thinner – I use acrylics so water – and scrub away. Try to go In the direction of ‘traffic’, in this case the airflow or the pilot’s feet
After a while the bare aluminium underneath starts to break through in a random pattern. Not unlike the intakes of that Harrier up there ^. You can just make it out on the leading edge of the starboard wing in my last picture
* I used a pump action gel spray, decanted a bit into a spare Tamiya paint jar & diluted it with water. Much easier to manage than a can of Insette Spiky