Would it be permissible to use the Water Wow! toy on Shabbat? Assuming that it is, at the very least, uvda d’chol, what are other issues are there?
Here’s a description of how it works:
The basic concept is that each page is “laminated” with a very thin sheet of paper – almost like tissue paper, but more durable. When the paper is moistened by the water in the pen, it becomes see-through and reveals the picture underneath. There’s no chemical reaction involved.
Had it been that the water was interacting with some chemical to create the color, it obviously would be forbidden. As it is, you are just revealing a color underneath. It would be like wiping dust and grime off the glass over a picture and revealing the picture underneath. However, your case is definitely one of u’vda di’chol – the experience here (as opposed to the grime on the picture) is one of coloring. Therefore, it would still be prohibited to use on Shabbat.