So I made some colouring-in cards:
They're printed on a lovely thick uncoated paper, that has just enough of a tooth for pencil crayons, and goes well with the squiggly dip pen lines.
By happy accident, the ink is not only waterproof, but slightly water repellent too. So paint right over the lines - they tidy themselves up.
I tested pencil crayons, gouache and watercolours. There's a bit of buckling from the wet paint, nothing that a night under a phone book won't fix. (Because you do still have phone books, right? For this sort of thing.)
I've listed these in my Etsy shop and some of my local shops as a set with envelopes. They're thick enough to work as postcards, but you'll have to draw the lines on the back yourself. While my original plan was to make postcards, I'm not sure anyone buys stamps and posts postcards these days, and great-aunt Edith might feel a bit miffed if the card you stick on her present has postcard lines on the back.
But let me know what you think; if you'd prefer to buy them without envelopes, and a touch cheaper, it can be done.
Oh, and did I mention that they're sort of educational, too? I mean, they're more or less made-up insects, but all the bits are in the right places.