Product review (4 stars): These work, but some words of caution for those unfamiliar.
These work as advertised, I'm using them inside sealed clear plastic totes to show when the silica gel within needs regeneration.  They change from deep blue to pale blue, finally will start going pink when humidity is well beyond what the spot shows.  However, they will go back to blue again when the humidity drops below indicated level.

I have ~10-15 cycles so far, and they're still working.  Compared with other hygrometers, these seem fairly accurate.

Using similar indicators in the past - be aware, direct sunlight, exposure to liquid water, and extended time at 90% or greater humidity will cause them to stop working eventually.

Also be aware, the chemical that causes these to change color with humidity is toxic.  Minimize handling and don't have in direct contact with foods or items to be consumed.  As far as I'm aware, having them in the same container at ambient temperatures - without direct contact - is fine.

I would give 5 stars if they had a "reference" printed on the card, as when they gradually shift, it's hard to discern the "lavender" point over time.  I have other cards that have a comparison block printed for this purpose, which would be very nice to have.  My solution is keeping a fresh card with silica gel inside a sealed bag for comparison.