>>10148137AK made a book for planes and one for tanks. It's not complete, but you'll find all the most important ones.
If that fails, search on forums for the federal standard code. For example, FS34159. Every screen displays colors differently, but it's the closest you can get to the precise shade.
Finally, some companies sells color sets for vehicles such as pic related, with the exact shades for every nation and theatre of war.
