Thank you Teal
I can't get my camera to differentiate between the grey and white but you can see it with the eye...
This is the way I do the underbellies of my aircraft and skimmers.
The whole model is primed in matt black then the upper side is painted down to the point you want the change in colouration to begin - I use low adhesive decorators tape to mask of the lower surfaces as I use a green primer as a base for the upper surface.
I then mask off the upper surface and spray the underbelly in a light grey (car) primer. I have found one which is almost the old Fortress Grey.
Paint the underbelly in Fortress Grey (I use Vallejo Stonewall Grey (Game colour 049))
Lightly load a small strip of sponge with the grey and use it to break up the join between the upper surface and underbelly.
Lightly load a sponge with white and lightly dab random speckles of white over the underbelly and onto the join between the upper surface and underbelly.
Add decals as required
Wear and tear:
Lightly load a sponge with matt black and very lightly mark soot trails from engines, grills etc in the direction the vehicle would travel in ie front to back.
You can also use a flat headed brush again lightly loaded with black and draw it from the front edges backwards to give a dirty appearance but I only use this on my flyers rather than skimmers.
Lightly load a sponge with matt black and apply to sharp edges as a base for paint wear.
Over the black but slightly less coverage, use the same sponge to dab silver to show the paint wearing off the metal surface.
I use Burnt Umber oil paint. You can pick up a tube at most craft/painting stores and it will last for many vehicles - (I still have not used 1/10th of a tube on all my vehicles including IG and DA).
Thin using turpentine until creamy and then, using a paint brush only for the oil paint, paint it onto panel lines where you want the dirt to come from - engines/landing gear/hatches etc.
Allow to dry off for 10 mins or more - depending on how thin you mixed it - then use a flat headed brush to feather the paint towards the rear of the vehicle.
Note: the oil paint will have to dry for at least 12 hours or preferably much longer before it is fully dry so move on to another project while you are waiting.