I'd start off with a dark gray, almost black base coat. Then, take PollyScale Light Grey Undercoat and mix up three bottles. One bottle is the Grey, one bottle has 3 drops of white added to the grey, and one bottle with 3 drops of black added to the grey. Thin appropriately.
Now, the fun part. Spray the entire model with a couple of light mist coats of the straight grey, so as not to build it up too much.
Next, use the 'lighter' mixture and using a 3x5 card with panels cut in it as a mask, spray a variety of the panels on the model.
Repeat this with the 'darker' mixture.
I use a variety of rectangular shapes to avoid that 'checkerboard' pattern.
Try this method on something else, first until you get a feel for how it works.
Let each color dry thoroughly before adding the next.