If you get true purple phenos, they start going purple right from the onset of flowering.
The Devil Cream I grew had purple calyxes from day one of flowering.
It should be purple and I would drop Sweet Seeds a line asking about this, but unless the seeds you got came in the breeders pack, you probably won't have much of an argument. Either way, they might be interested to hear about it in case it was a particular batch of seeds.
With Devil Creams parents being Dark Devil and Black Cream, you would think purple would be a no-brainer.
However, Dark Devil is a cross between a green mother and a purple father, so it could just be a throwback to recessive genes.
Anyhow, colour does not affect potency and your buds will still be delicious, even though they are green!
Hope this helps, Organoman.