Depending on genetics some strains will have some reddish tinting to areas other than just the leaf stems. But if it seems to be a problem then it could be one or several. A nitrogen deficiency can cause red stems, but are the rest of the symptoms of a nitrogen deficiency there? It could be a phosphorous deficiency too. Potassium problems can cause red stems. Can you find any symptoms of any of the above besides red stems on your plants? If not then I would say it is genetics but if you do then you know what your problem is by seeing what problem each will cause in what way where on the plant and what else would go with it and then applying the solution.