Purple undersides of leaves is a sign of phosphorus deficiency - typically caused by cold because it prevents the plants from taking up P.
When I give my seedlings the cold treatment, purpling of leaves and stems is a sign they have been cold enough.
If it isn't cold, then some other reason why phosphorus uptake is not happening... deficient or maybe unbalanced ferts.
I love this explanation of phosphorus uptake factors, that is posted at Fusion's site:
http://www.selectedplants.com/OrthoPhosphate.htm