Verkkowhat is an evaporation line on a pregnancy test? How does an evaporation line compare to a faint line on a pregnancy test? How can i avoid an. Verkkoon a blue dye test, the line can appear as gray or even a lighter shade of blue. For some reason, anecdotally, many mamas find that blue dye. Verkkohere's how to tell the difference between a true positive pregnancy test and an evaporation line, plus tips for avoiding an evaporation line on blue dye or.