When cold this mug is completely Muggle worthy, no magic to be seen. However pour in your tea/coffee and watch the magic happen as a creature appears through the keyhole.
When cold/room temperature: blank keyhole under the text "muggle worthy"
When hot drink added: creature eye appears in keyhole and text disapears
Official Harry Potter Licensed Merchandise
(Note: whole mug does not change colour - picture depicting two colour variations)