Product review (3 stars): Convenient but not very easy to read.
I like having a simple way to monitor the humidity in small containers and these cards fit then bill. Unfortunately reading the colors isn't so plan and simple. The colors aren't dark blue to pink when in use. The cards display a pink and lavender shades and it isn't that easy to judge the fine color differences right away... Is the dot pink, lavender or a shade in between? Its difficult to say when all the dots are very much alike. It's only very easy to read if it's very dry or damp or changes quickly. It takes some study and getting used to understand it's actual readings. Lighting also makes reading different from place to place.

Over all they are nice to have. But don't count on hyper accurite readings.

I recommend comparing the cards to known humidity reference to understand what each subtle color shade means.