Why would you want to do that?
When they get warm and can transfer nutrients the stems will just turn green again. Some do have purple stems but that's just a natural trait of the plant.
You don't want to get them cold man, unless you want to stunt growth for a few weeks. What I have found that works best is a root temp at 70 degrees. I heat them with a seedling mat. Then your Highs around 85 degrees. around 50% humidity and fresh air after they are 5 to 6 inches tall.
Good Luck