The food coloring isn't moving up into the roots, it's staining their surface.
You can eat food coloring but it is not FDA approved for SMOKING... scary.
You do see other fall foliage colors like reds and oranges in strains such as Soma Rock Bud.
If you want to go down below 60, do it during only the last couple days before harvest when it won't make a difference.
I'd keep night temps in the mid 60s during the final flush period. You are breaking down chlorophyll then anyway and that exposes other colors/pigments that the plant has present.
If that strain does not have the genetics to make other pigments, nothing you do will turn them purple, short of a spray can. (don't do that lol)