Why are stems changin color?

Two plants in third week of flowering, and i'm wondering why the stems are turnig that red/purple/brown color.... its kinda hard to see in the pics but the whole stem holding up the leafs have it.

its nothing to worry about, could be due to cooler temps in the room.

I noticed my plants doing that more than usual a few weeks ago. I think mine was caused by a deficiency. Stems started turning red, and then the leaves started yellowing. After some feeding the stems got more green back and all is well. Still a lot of red, but plant is healthy.