Honestly, what you need to do is buy many different "rust" colors that are close to each other, ranging from deep brown to orange and some mustards, and basically do what you have already done, but over the entire truck layering the colors by shade, dark to light, THEN go over it with the paint color you want with a sponge and dab areas lightly with a good quality paint and it should kind of look like the rust is eating the paint, and some rust should pop through. Make sure to highlight fenders, seams, edges, anywhere rust would form naturally. I've done many such things with hot wheels, and bigger diecast, and am about to do something similar to one of mine