I am somewhat familiar with the arguments about horseradish vs lettuce (or other stalks that leak white fluid) for maror. Disregarding that, assuming you follow the tradition allowing horseradish as maror, someone told me it has to be the "red" (or purple) kind of horseradish, with beet juice mixed in to color it. They gave me no explanation at the time they said this.
Is it true? If so, why? If not, any guesses why someone might think this?