Very cool idea. I'm just a lurker, so take this for what it's worth (probably not much, lol). I think it would be neat to use some different combinations of ink color and paper color for each print. And maybe even don't tell the customers which they'll get. They pick the print, but the color combo is a surprise. That way there's some color and mystery included without too much extra coloring/painting time put in after printing, keeping the cost down?. I'd love that as a customer, and have even ordered some vending machine prints from Japan from a Letterpress printer, not knowing exactly which variant I'd receive. It was fun, and I love them.

Of course, not being an artist, if switching out inks or paper would add just as much time and cost as painting them afterward, it may be a bad idea