Purple stock and stems

So three out of four plants have purple stocks and stems some even have purple main vein in the leaves. The last one is green through and through. The plants are on week four of veg and the room only gets to about 68 at night so I didn't think it was from the strain itself.

Is this an issue? The plants look good right now

are they on a cold floor? or any chance of the root zone getting too cold?

The one plant may be asking for P/K. Do you notice any other problems on the topside of leaves?