"Grey" and "water" are both valid words that compose in English. And yet "grey water" is a specific thing. If you showed up and started shouting, "But it's wa

"Grey" and "water" are both valid words that compose in English. And yet "grey water" is a specific thing. If you showed up and started shouting, "But it's water! And it's grey!" about some water that had been contaminated with fecal matter, then you'd rightfully be shut down.