i have seen stones turn color as they melt. the sun may also be having an effect. i say you need to do your own control. someone elses flush is not coming from you. my first stones melted into a dark red. what oil is dark red, you may ask? hemp oil, from the plant, not the seed. it is not only dark red but one of the most sticky substances on earth. i had smoked hemp daily for years.
this still doesnt answer why sometimes stones form and sometimes they dont.
red palm oil is already red.
maybe a color is the wrong direction. perhaps some other marker can be put in the oil, like a colloidal suspension. i am no fan of it, but titanium dioxide comes to mind. for the sake of science, of course.