Okay, they are not actually on Altoid tins. I bought some generic blank ones instead, off of ebay.
And I decided to make them not-blank.
I used Daniel Smith Watercolor Ground. I prepped them first with multiple layers of Ground, getting a very nice textured surface, then painted with watercolors. Afterwards, I sealed them with an acrylic varnish spray (many layers).
I only spent about two minutes for a rough sketch on each of these, and then I started painting. They were relatively unplanned, very messy, and a whole lot of fun.
Take a look at a time lapsed video with commentary for Firebird to see how it was done:
This second video isn’t as informative, but it does let you get a glimpse of how the Blackbird piece looks under light: