As already mentioned, most "darkening" is a pre-shed change, but some BPs do have light and dark phases unrelated to shedding. Some Orange Bellies get really dark over time, which appears to be an ontogenetic melanism. Many other BPs appear to demonstrate a more rapidly transient color change that appears to be a photoreceptor mediated neurohormonal manipulation of chromatophores. I've noticed this most prominently in my GHI, but I've seen it in other morphs as well. If your snakes aren't on a photoperiod and in relatively clear enclosures, you are unlikely to see this change.