It's my understanding that stem color is a result of several factors. Genetics is probably number one factor, then deficiencies can cause color change, and I've heard that temperature can affect stem and leaf color. I would say that if yu are noticing a sudden color change in one or more of yer plants stems, and yer temps are ok, then keep a close eye on the leaves for changes that would involve a deficiency. I would also check the ph of the water and or grow medium. If yer in soil, make sure yer water is ph'd right and then check the runoff after watering to see if there is a big difference. a small difference isn't an issue but a big one could mean that yu have too much of one chemical in the soil that is throwing off the ph, and that will definitely cause lots of problems