Well, goodness me, you learn something every day, what will they think of next.
Just checked this out online and you're right, you can spray paint your lawn green. There appear to be two types of products available, one dye and one pigment based. The latter professes to last a lot longer than the dye one. Trying to find out quite what ingredients are present isn't easy - they claim its good for the environment, but this appears to be simply because you will/may water your lawn less to keep it green. I'd argue that all the paint will do, if you water less, is conceal the fact that your lawn is dying/dead and may not recover at all with no water in hot parts of the world. Certainly ammonia seems a large component. The pigment based one, exposed to 'thermal conditions' (whatever that means - sun?) will give off Co2, so hardly good for the environment.
Both products carry warnings regarding eye, skin and breathing exposure, so seems you need a bio hazard suit to use the stuff, really, and it should be done on a windless day when the blades of grass are completely dry. The pigment based one seems to have been originally approved for professional rather than amateur use.