i did a whole bunch of GDP out door last year. clones i got from a friend. and i wasnt mad at the friend or anything but the only part of these plants that went purple was the little bit that got left behind after harvest was done that just stayed on and matured. not one bud went purple on me. i called it grandaddy greens lol but in a grow room im not sure what to tell you. im an outdoor guy i know that temp does have a lot to do with it so you may not be running at a low enough temperature. i would turn my thermo to to like 76? (but dont take my word i would go into the indoor forum and ask. they will be glad to help.) for the last 2 weeks and that would tell them to rock up and go purple. but also i think that some strains just may not be the greatest cuts and didnt develope their purple genes like the ones i had because i had a cold winter last year that came earrly and i didnt have much purp in the crop which was a little bummer but i made it work. best of luck! hope i could help you a little