The biggest challenge will be to find a paint that is both long-lasting and ecologically friendly.
Acrylic-based products are generally considered to have a low toxicity, but they are polymer based and tend to flake over time, especially outdoors with abrasion and environmental elements. Latex-based paint
should be fairly similar.
For durability, paint designed for asphalt and
concrete (the kind that is used for marking street lines) will give you the best results, but I don't know how non-toxic they would be. But perhaps, a more toxic paint that stays put and lasts a really long time is better than something more eco-friendly that only lasts a few years and leaches in your garden. You'll have to read the safety notices and ingredients list very carefully and do some research.
I'd suggest experimenting with different techniques with a variety of pigments, make a bunch of samples, and leave them outdoors for several months (preferably a whole year) to test for colorfastness.
Otherwise, Anne's idea of making mosaics is probably a sound one: ask around for damaged pottery or glass, break that into bits, and compose your scene in concrete. I'd probably do it in small sections and then lay them out
side by side, so you can work at the project gradually. Mosaics like that can last a really long time in all weathers. But concrete is a fairly non-sustainable material, unfortunately (there are better options starting to be developed, but nothing that is sold in small batches to individual customers as far as I know)