Seems like there ought to be a lot of things that would color/stain porous concrete. Liquid shoe polish (do they still make that), strong coffee, diluted paint, aniline dyes, furniture stains. Why wouldn't any of those be a possibility? Come gather up my grasshoppers! Right now they've been muching on some of my plants and their normally tobacco colored juice is a really attractive shade of red grape. I'm sure it would stain concrete! You'll need to catch a lot of them, but I wouldn't mind. Come take them all if you want
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