Hi,
Perhaps the lowlanders, which get less light for the most part,
are getting burned by the sun? Or the lower temperature is causing
the plants to turn this color... N.gracilis can become completely
red in cool/cold weather as can N.mirablis. N.gracilis (negro purpurea)
can, supposedly (I haven't let mine get cool), become very, very
dark to the point of appearing black or blood like.
N.ventricosa is a highlander and, generally, gets more light
and will like cooler temps so that is why it's not affected either way.
Dave Evans