Since you are interested in protecting your reputation, it's important to ask yourself what will earn you the most respect from your colleagues and managers - placing blame on a junior team-member, or taking responsibility for your team's mistake? I can recall several instances where team respect went up significantly for a manager who took responsibility towards upper management for one of our own failings.
It's also important to remember that, realistically, a manager/senior is in some ways responsible for the actions of their team, or juniors. While ideally your junior would know what is acceptable to communicate to a client, you were aware that he is a junior and didn't clarify (presumably) what is acceptable to communicate, or if you did communicate this, it wasn't in a way that is clear to his learning style.
In either case, fault can be shared in this mistake, so I advise you to, while not taking total responsibility for the mistake, don't "throw your colleague under the bus," as it were. This won't win you points with him nor your managers. Your best bet would be to think hard on where the failure point was, what could have prevented the mistake from happening, how you can adjust going forward to ensure the mistake doesn't happen again, and what sort of damage control you've implemented/plant to implement.
If you're explaining why the client expressed concerns about the team, you could say something along the lines of "(colleague) and I are aware of the client's concerns. The issue came from a miscommunication between (colleague) and myself about the points of contact with the client. We've worked it out that I would be the sole point of contact going forward, and have already discussed this with the client, who seem happy with the solution."