So for the Sunn Model T it was a bit of a fluke. I sponged on some aluminum black to add texture and depth then sponged on purple gel food coloring. To make it dry faster and get rid of tackiness I baked it briefly for 10 minutes at 150F and it turned blue and purple.
The Westwood is just green gel food coloring spread over an etched enclosure with enamel filled in the etch, I then remove the streaky food color lines by dabbing the enclosure with the sponge. I got rid of any tackiness by using a hair dryer, I tried a oven at a low setting and green turns sort of brown, which was not as nice. I then put a enamel crystal clear coat on the enclosure on the right and flat on the left. I will say in the future I would put a thin coating on enamel first then food coloring the food coloring doesn't stick super great to aluminum. Definitely worth playing with as an easier finishing method.