Yes if just the stems stay purple I'd say you needn't worry. If leaves start going purple than dum dum dummmmmm bad shit happens and people die! usualy
what kp said is the case..it could also be overfeeding..i have heard with certain nutes you can overfeed an cause the main stem to harden an stop growin...
Are you serious? did you see this? i herd that didn't work... I read to make a plant purple keep the temperature below 75 F or that its a sulfur deficiency.
your right...its also strong gentics...plants that are bred for color maintain it mostly...but yes its also when its cold or sulfer....but with my exp if a leaf stem turns purple in flower that leaf is going to die in a few weeks...
it could mean nothing..just wait if you seen any other leaf problems starting then worry..problems include leafs curling,,tips burnt,,leafs having dead spots on them...if these dont happen then grow away...