tho its normal for some strains to get purple stems ..
but it can be a sign of some def. cant remeber witch tho .. for some reason my memory is pretty bad .. just cant remember why tho..
cal/mag or iron maybe .. look it up .. but if the plant look nice and healthy and grow you schouldnt worry to mouch ..
I have 3 LSD seeds and one Voilator kush going right now .. and the VK have purple steams .. the LSD dont .. know the VK from last grow .. had purple steams ther aswell and no problems ..