the only think than you say with sense is the last one: completing campaing gives one type of paint.
BUT, there is a better idea: paint unlock system.
16 diferent textures, 4 special paints.
every paint is unlocked with archivements.
- for free 16 paints: get rank 15, get rank 10, get rank 5, get rank 1, get 100 wins, get 1000 wins, spend 200 tokens, 500 tokens, 1000 tokens, complete normal campaing, hard campaing, insane campaing, have 3 mech over 700hp, max one item, get 1000 tokens.
- for special paint: win tournament in solo mode (first 3), win tournament in clan mode (first 3), be suporter and be premium (or beta tester).
where put the paint system? easy, on the upgrade screen like a pop-up menu.
in this way you dont need add items or make chains to put em on inventory, only make the programing way to make the menu, and the option to paint any item selected for upgrade.