I have bought coloring books for my son. The book has pages of pictures what turn to color after you brush it with wet sponge ar brush. How does it work?
Also, some of them are single use only – you get them wet, they stay colored. Other (more expensive ones of course) dry out and you can re-wet rhem again after some time. How does thaaat work?
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My Little Mermaid one I had as a kid had a very fine and widely spaced dot matrix over the image and the dots contained water soluble dye.
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