I say this is a fackt, all strains nearly 95% strains and plants yu can turn fall color's, yu have to give them cool air, not just cool, nearly cold air. and the plants starts to turn oranges, purple, reddish, dark red, nearly black, some blue some light and deep purple, different onrage shape's like in Autumn Colors in north side of planet
never know what turns more purple, and what turns more onrages and what turns more red shape's!!
But There is purple strains, like Purple #1, Purple Haze, Purple Power and Purple Bud what is very potent, easy to grow and very strong and nice up high
it's 80% sativa with skunk genetics i got those seeds a lot.
So these strains will gonna changes the color's more easily than non-purp strains. sometimes they just need little bit cooler air at night and at the day time, but sometimes nearly cold.
if yu live north side of the planet
heh heh.. then it's easy in winter time, when yu got -5c to - 35celsius , just open the near room window like 10-25min's every day. 1-2 times. If Yu keep the window open like 10 min's do it couple times at the day, but if yu keep 20min's do it just once, or give it cool could evening air if yu dont got so could around, and good thing too if yu spray some cold water little bit every day.
and purple strains got better genetics to resist coldness and cold air, so thats why they are best to do it, and if yu use sativa kind a strain then yu not need so cold air to achive colors, coz sativa like warm and enjoy heat ( not too much heat ) but indica is stronger to against to cold, so indica can take more cold and cool air.
i hope this will help.
ONE MENTION!! the colours start to shown only in flowering stage, not in the begin, but in the mid flowering and till to end.
SO in vege yu cant get colours. SO DO IT IN THE FLOWERING!