If you've heard of milk paint, it's probably because you're a furniture geek: Maybe you've fallen in love with a worn Colonial-era chair, the way the colors of paint jobs past peek through the top coat along its edges and corners. That's milk paint doing its thing, the way it's been doing it since ancient Egypt.
And the smudgy wearing-away is part of why it's prized; the whole "shabby chic" trend, which spawned an industry of intentionally distressed furniture and driftwood signage, is based on the milk paint effect.
Milk paint doesn't go on perfectly even; it's mottled and opaque just in places, like a watercolor got married to an acrylic. Those who love it prize this irregularity, which looks as beautiful as lead-based paints but without any harmful chemicals, toxins, or fumes. Moreover, milk paint won't chip, instead wearing or cracking where it's thinnest (or where it gets petted most frequently). Like so many natural materials, a milk paint finish gets prettier and prettier over time.
I was recently re-introduced to milk paint when a paper bag of dry blue powder landed on my desk, a gift from the Old Fashioned Milk Paint Company. You can find powdered milk paint pretty easily these days (the Old Fashioned Milk Paint Company offers it in 16 beautiful colors), and all you need to do is mix it with water before painting it on. It even comes in cans, if that's one step too many for your taste.
But since we're here, on a site that celebrates the hand-crafting of anything that could be otherwise store-bought, it's especially important to note that milk paint is something you can make! The basic process is to curdle milk (using lemon, or lime, or white vinegar) overnight, and then add pigment to the whey. The proteins in milk called casein act the same way oil does in an oil-based paint (or egg does in tempura, and as polymers do in latex-based paint), suspending the pigments.
The following recipes is based on Martha Stewart's—hers is one of the simplest I've found, and it produces a very easy-to-use result. If yours turns out lumpy, you can add borax or bicarbonate soda to smooth it out. Just be sure to use it the day you make it (and therefore only make it in the quantities you need) because, like milk, it will spoil!