Interesting. There is a well-documented phenomenon that many plant's colors darken as temps drop. Cephs definitely have this propensity to darken as it cools but as many have pondered, there appear to also be other factors which we're unable to regularly duplicate. People on the VCPS listserver recently had a discussion on this topic and could come to no solid conclusions.
I have a 'Czech Giant' I received a year or two ago as a tiny plant. It has been putting out adult pitchers for at least the past 6 months. While the plant does not grow as fast or as large as my HG Cephs, each pitcher it produced this year has been almost black, while HG Cephs & HG Ceph progeny in the same tank have nowhere near the dark colors.