One suggestion I'd make to the rules is going to five shades instead of three. That way you can have a highlight, the "base", and then three darker ones to encompass shadows, bolts, and panels lines on vehicles.
Not going to lie, I procrastinated on this challenge until it was too late to get any of the background fluff for the designs really finished. So here's what I've got https://images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/f/6fd41f46-7797-4e5c-8e77-c954fea51731/ddf8dmp-928c76fa-4e76-4720-8917-258457...