my STEMS on my FAN LEAVES are PURPLE how come?

the stems on my fan leaves are purple.they are diferent strains so I know its not just a strain thing because they are both doing it and I've grown these strains before without it happening. the only difference is the temperature of the air I'm blowing on them.the air is cooler than normal. I know that you can turn any strain purple by lowering the temperature of your grow room but this is just the stems on the fan leaves.
the other thing I thought it might be was my flood and drain hydro system was accidentally feeding them during the dark period while the lights where off,and its not supposed to feed them at all when the lights are off so I might have burned the roots a little bit. maybe that is what caused it.
if anyone has any ideas on what caused this let me know

what are ur temps at? yea cold temps can do that...

cold temps at night or genetics m8 hope that helps

Temperature drop significantly at lights off?

could be a nitrogen deficiency