Hello!
After I saw your interesting question I had to try by myself how this would works. And I found some interesting facts:
First as it says you can use any kind of liquid on the Bath Tub like, Wine, Beer, even Urine, yes Urine XD.
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But it will always turns into a blue color and when you click on it, it will shows that is water
.
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So as a funny fact we can take a "very inviting bath tib filled with a fake water" and we will never know.