Why does the space between ceramic tiles on the floor (where the grout is) often turn almost black when most dirt outside is a light brown? If the dark coloring comes from dirt transferred by shoes, shouldn’t it turn the color of light brown dirt?

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Why does the space between ceramic tiles on the floor (where the grout is) often turn almost black when most dirt outside is a light brown? If the dark coloring comes from dirt transferred by shoes, shouldn’t it turn the color of light brown dirt?

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The black is the mold and/or mildew that grows there after it gets damp enough, and not just dirt.

Grout soaks up stuff like crazy, and fungus spores are just one of those things that can grow bigger and stronger with time.