Coloured Spots On A Ruler Defy Logic

Magic tricks performed with easily identifiable objects can be especially confounding. It's one thing to be entertained by a magician with fancy boxes, etc. but who's to say they aren't prepared in some way? On the other hand, when the items being used are familiar and not easily tampered with, to me the effect is far more mystifying. This video shows the magician placing a white spot sticker on one side of a standard ruler. The spot is then coloured with a blue highlighter pen. A second white spot is then coloured likewise, and caused to vanish. This spot is found on the reverse side of the ruler. However, by passing a pink highlighter pen over one blue spot, it changes to a pink spot. Confusion reigns as the spots refuse to remain constant.