Any question answered...

^^^ paint it with acrylics? needs to be something with opaque colour. or if that's too fiddly, a spot of decoupage-style should work ok, but not very durable. print on thin paper, if possible laser print or use inkjet inks that won't wash out easily, then use a flexible varnish.

I've painted on leather with acrylic artists' paints before now, seems to work.

If you want something hard wearing I'd dye it, or print on it with indelible inks.

Thanks all, extremely helpful. :)