I'm less concerned about changes in bark color than I am about leaves drying up and falling off daily. I would be concerned about watering - the leaf drop does sound like a water management issue. It could be not enough as well as too much. A lot depends on the size of the container compared to the size of the rootball, the quality of the potting medium and the weather........only you can decide if you are watering too much or not enough. JM's prefer their soil to remain evenly moist - not too wet and not too dry.

Bark color does tend to fade in summer. For example, the bark on a Coral Bark will be almost unnoticeable in its coloring in midseason (except for the very tips) compared to what it looks like in winter. And older wood - like the trunk - will eventually turn a brownish gray and will never become red again. The newest growth will always have the most intense bark color and that will be most noticeable in cold weather. That's just the way this tree rolls :-))