Some plants have red/purple stems naturally (genetics) and it can also be down to a lack of phosphorous, but this is (usually) highly unlikely, as all modern cannabis fertilizers have adequate phosphorous levels.
Either way, it is not something to be overly concerned about, but a dose or two of a P/K booster won't harm them, even though they are in veg and if it reduces the redness, you will know they do need more phosphorous and if it doesn't reduce the redness, then you will know it is all down to genetics.
Hope this helps, Organoman.