The three colours are mixed in the airbrush paint cup, but thinners (white spirit) are placed first. That is because  the first thing to reach the nozzle is then thinners rather than unmixed paint. The latter will block the nozzle before you even start the weathering. When mixing colours, start with the lightest and add the other colour(s) using small amounts at a time. In this case the orange was added to the thinners first (a small drop), the leather was added next (one brush load) and the black (another small drop) was last.

A 9v battery is used to turn the wheels while they are being sprayed with the underframe colour. By bringing the airbrush close to the target, only a small area is painted at a time, and using this airbrush the area covered is only the wheel.