I grew bc bud depot the purps a year or two ago. Didnt get any purple till late flower.
That said, it is pretty common for purple strains to not purple at all.
Good Luck
Unless you are growing a clone from a plant that turned purple at the start of flower then you can't expect yours to. I just looked up that strain and, from what I read, 9 out of 10 will turn purple. Your pheno can just be one of the 10% that don't get nice and purple. If the color is important to you then drop your temps to the high 50s - low 60s and you should get your purple
The emoji at the end of my comment was to show I was saying that as a joke. I was basically saying to drop the temp if "purple" is more important than growing it out properly.