Moss turns black (and hopefully dies), but ferrous sulphate stains concrete etc brown.
You used to be able to buy 25kg sacks of ferrous sulphate for less than £10, but then the EC stepped in, and declared it a hazardous substance. [Well, it certainly is, if you drop a 25kg sack on your foot, or give yourself a hernia lifting it in and out of the car boot.] These days, for the ordinary man-in-the-garden, it's only available in (say) 1kg bags, and costs around £10.