Counter shade with grey, brown, a camouflage yellow, or white, or all in that order. I have found using the final color lessens the overall effect. The grey would be primarily to dial back and refine the black basing lines.
Regarding the marbling thing, I have seen a model online where the builder used primary colors as well as white to cover the kit’s surfaces with tiny dots of color. It looked really awful at first, but when the color coat went on, it looked good. I guess it’s kind of like doing the same thing with oil paints when dot filtering - the vague color variations suggest detail and depth.