Color-changing can we do this safely at home?

I’m trying to help my little sister with a science fair project. Do you guys know of anything I can get that changes color when wet, either a powder or clay or sand? Or even a reaction that would take place that would change the material colors when water touches it?

Litmus paper?

There are a variety of natural stuffs that change color in the presence of an acid or a base.

Any (or most?) flower or vegetable with a deep purple color works. Changes to red or blue in the presence of acid or base (I forget which is which).

One recipe: Boil up some purple cabbage until the color all comes out in the water. Now you have a purplish solution. Drop some acid (e.g. lemon juice) or base (e.g., soap or lye) in, and see the color change.

You can do the same with purple iris flowers.

Not sure how you would distill this down to a dry powder in any useful way. You might be able to paint a piece of paper with the purple solution and let it dry, then dab a wet acid or base on that to see the color change.

Not sure what you would use that would change colors simply by getting wet. Perhaps a dry mix of litmus (or whatever) indicator with dry acid or base powder, that will change color when you wet the mix so they can interact.

ETA: There’s also a chemical called phenolphthalein, which any school chemistry lab would be likely to have, I think. IIRC, it turns red in the presence of a base, and colorless in the presence of acid. Is that available as a dry powder, or only in a solution?

Use a liquid-contact indicator like the ones used in laptops and smartphones.

There’s also a tuff called Shakudani stone, but I’m guessing that’s going to difficult to get a hold of.

Turmeric powder and detergent powder. When dry, it will be the same colour as Turmeric (a deep yellow) and when wet, the colour changes to red (since turmeric goes red in alkaline pH).
Im not sure if baking soda is alkaline enough to substitute for the detergent powder, but if it is, you have an entirely food grade colour change reaction.

Another food grade option is citric acid and dried and powdered petals of purple iris/ clitoria ternatea/ any other anthocyanin rich flower as mentioned by Senegoid above. In its dry form, the powder mixture should be a dark blue colour, when hydrated, it’ll become pinkish red.

Simplest solution: anhydrous copper sulfate. You can buy copper sulfate by the kilo it at any major hardware store or gardening supplier. Here’s a videoexplaining how to make anhydrous copper sulfate from it using heat gun. You can also convert it by putting it an regular oven at >220 C/450F for about 2 hours. Just put it in and wait for it to turn white, then take it out and store it in an airtight container.

While I wouldn’t advise letting young kids do the actual conversion, any adult can do it with safety. And the resulting product is essentially non-toxic and changes from white to vivid blue as soon as it gets wet. Far more spectacular than common acid-base indicators like litmus or cabbage bye.

Cobalt chloride is used as a moisture indicator on desiccant pellets; when dry it’s a vibrant blue, but when damp/wet, it turns pink. To restore its dry/blue state, you bake the pellets in an oven at ~400 degrees for an hour or so to drive off the moisture.

I wonder if a chemistry supply company can sell just the CoCl powder?