A lot of subassemblies were painted multiple colors ie, inside the instrument bezel using a simple mask. What were they typically made from? I can’t imagine cardboard would hold up for very long. Though for my purposes that may be the best option. My uses will be in the low single digits not the hundreds.
Depends on the supplier/maker and the specific part I would guess based on a couple I’ve seen. One was a series of brass or copper masks in a series. One for each color in sequence
You ought to find someone that was/is a shoe salesman all their life and ask for some shoe box lids to use or their wife might have some BonBon lids… 
Wait, I really DID sell shoes while going to college…1968-73!