Howdy Johncena, I have noticed the 'purplish' coloration on the stems of my plants on previous grows. It seems to only appear where the light directly shines on the stem, in the past the color stops where a fan leaf shades the stem. In my opinion, it is a natural reaction to the direct light, sort of 'tanning' in people's skin. I have not noticed ant detrimental effects from it, not like the light 'bleaching; of the upper leaves that are too close to the lights.