Stems turning purple?

i think i have seen this problem on here somewhere but i havent been able to find it, so i was just wondering if the stems are turning purple what type of problem this is? I just noticed it today after watering without nutes on tuesday. there are parts of the stem that are still green too

Nitrogen deficiency.

Most of the time it's a genetic thing but if it were any deficiency it's usually phosphorus.

how old is the plant?
What are you growing it in?
WHat strain is it?

What are your temps, nutes, lights...etc...

HEY i have bubba kush and hindu kush clones in a hydroponic tote.. the stems on them turned purple as well.. i transplanted them from small soil cups to hydro with hydroton and rockwool

the plants are a northern lights strain, about 2 months old, growing in soil. i dont have a thermometer (though the thermostat in the room is set to turn the heat on if it goes below 65) and i rarely use nutes because all i have is MG and im waiting to get some nice fox farm stuff. I have 6 CFL's at 26 watts each (that's 125 or 150 watt equivalent) for 5 plants.