as stated, it's a genetic thing... usually the last 1-2 weeks true purple strains will turn purple on their own... if you cut them early they will still be green.... certain strains like cherrie pie (also purple) are also know to throw seeds in the last week of flowering if at all stressed, so if you go too long on those and try to get them cold (shock) it may be a bad thing... I've seen several purple strains throw seeds from going too long... usually you'll have many seeds at the bottom of the plant, but almost none at the top...