Yes, do the trouble shooting form found in the "Plant Problems" forum.

Quote Originally Posted by orion420
Darkening of the stems and leaves, while looking exotic, and advertised in some strains, is usually caused by nutrient deficiency. You may need to increase the ratio of nutrients you're giving to your plants. I imagine if you post what you're currently feeding them someone who has experienced this can solve your problem
Agreed, folks say purple stems can be normal. That's not if all of a sudden, in a matter of days, you look and say "Wow, those have really turned purple"! They/it are/is trying to tell you something. pH lockout, flush needed, and/or giving nutes may be in your near future.