60g of Ferric Ammonium Citrate (brown) and 30g Potassium Ferricyanide, which when mixed with water make 500ml of cyanotype solution.
These are light sensitive chemicals that can be painted onto fabric or paper. It starts off mustard brown and when exposed to the sun turns dark blue, due to UV light. The chemicals are then washed out and areas that were not exposed to the sun revert to white.
Works well on natural fibres, particularly cotton and silk and is permanent if applied properly.
Often used with leaves or plants, but you can use almost anything to block the sunlight from fabric/paper. There are lots of ideas in this 12 page cyanotype booklet, which also has video links demonstrating the process and some of the exciting effects you can get.
Please note that this is brown version of the Ferric Ammonium Citrate, not the green. It works just the same, but needs a slightly longer exposure time.