I remember a grade school science experiment where something about moisture or humidity caused a color change from blue to pink (or vice versa?).

I think the important bit was cobalt chloride. Doesn't sound like an ingredient in hibiscus tea though!

silica dessicant. chemists use it in their dessicators to keep things dry - pink is dry, blue is 'full' (i think, it's been 25 years...). i think it's a low level cobalt doping and as the silica matrix swells with water absorption it changes the UV absorption and the colour changes. or something like that.

That might be iron actually, somehow. I once cut a banana with a carbon steel machette and it turned blue.

If you pardon my intrusion, may I inquire why were you using a machete to cut a banana?

It's been a couple years (or more), I think I was just fucking around with a newly sharpened machette. Also I never managed to replicate it, though perhaps for lack of trying.

The water in your mom's place is probably a bit acidic if I had to guess, and the filter removes the ions

Shouldn't it turn red if it's acidic and blue if it's basic?

Yes.

Also, it is my understanding that calcium in water (which is a normal part due to the water taking up minerals in the ground) is making it slightly basic. I don't know enough about chemistry in this area, but I think the calcium in the water is mostly calcium carbonate, which definitely is basic[1]. So it's no surprise that water taken directly from the tap is making the tea blue, but after removing the calcium carbonate (maybe as part of a removal of any salts) that the tea remains red. I think this hints that an ion exchanger[2] is being used in the "very good water filter" e40 is using.

But removing calcium from water (along with other potentially beneficial minerals) is a bad idea, because we need it for building our bones etc. Hopefully those using ion exchanger are still getting enough calcium from other sources. But it's long been said that ion exchangers should not be used to "filter" water.

[1] https://en.wikipedia.org/wiki/Calcium_carbonate [2] https://en.wikipedia.org/wiki/Ion_exchanger

PS. you can put some drops of lemon juice into your tea and it will shine red :) (and have a nicer taste)

There are lots of people that love to mock those that prefer bottled water. https://xkcd.com/1599/

I’ve lived in lots of areas were the water burned going down. One place were water was just brown. Filtered leaves it weirdly bland. But acceptable for coffee / tea.

Hibiscus is a litmus indicator. Blue means your water is basic. That specific thing is how I make blue hibiscus pancakes.

Many flower petals can be made into a pH indicator.

Wikihow: How to Make an Indicator for Acids and Bases Using Hibiscus Petals

https://www.wikihow.com/Make-an-Indicator-Using-Hibiscus-Pet...

You can also use red cabbage water

Litmus itself comes from plants. (Lichens)

There could be something in the filter. A good test would be to make one tea with distilled water, and another with distilled+filtered water. Compare the colour.

Since nobody has linked to that yet: anthocyanins are sensitive to pH and can be found in a lot of things: https://en.wikipedia.org/wiki/Anthocyanin

This. Should have called a botanist not a microbiologist. That just looks like red cabbage was in there, shame there is no talk of other ingredients.