Totally light related, with a splash of nutrients. My mother colony way aways red, the frags greenish until they moved up. The Acropora below the RP is a Millepore, of course.
In my experiance---Frags tend to be red & green after they start to encrust. As the coral grows the only green will be at the base.......as it tables out it's all red with white tips. The coralites can be pink/red to all red depending on lighting and nutrients. Polps will be all red.