A Magic Mug is a coffee cup that has a picture behind a material that is black at room temperature. When hot water is added the black color turns transparent, revealing a picture or writing underneath.
What is the status of using such a mug on Shabbos? Is there an issue of writing or perhaps the changing of the coloring creates an issue?
This sounds a lot like those Transitions (R) lenses that turn dark in the presence of UV rays. Ovadiya yosef says that making this transitions happen on Shabbos is permissible; it is not dyeing.
The fact that there is hidden writing behind the part that changes color is irrelevant. See below for the source for the transitions.
http://www.halachayomit.co.il/EnglishDisplayRead.asp?readID=3384
