A lot of strains with purple genetics don't really show that purple until last few weeks. What strain is it? Given your feeding regimen, I think this is genetics.
Looks like a good normal late flower fade, maybe a touch too early but not something you should try to correct at this point. You'll have really smooth smoke!
BTW, the late flower fade differs by strain so don't be concerned if this looks a little different than other strains you've run.