I don't agree with your comments about arrogance. You can be right and strongly opinionated without being arrogant. Junior or senior -- making anyone, boss or not, look bad anywhere, but especially in front of a client is a perfect way of finding yourself at the business end of some pretty nasty social dynamics.
Practice voicing your opinion and arguing for it in a way that isn't confrontational. 9 out of 10 times, it's as simple as acknowledging the other person: "I see where you're going with this, it's a good way to solve A, B and C. But when we get to D, I fear X will bite us in the tail - Y can avoid that.". This also avoids the stinging embarrassment of being arrogant and later being proven wrong.
Practice voicing your opinion and arguing for it in a way that isn't confrontational. 9 out of 10 times, it's as simple as acknowledging the other person: "I see where you're going with this, it's a good way to solve A, B and C. But when we get to D, I fear X will bite us in the tail - Y can avoid that.". This also avoids the stinging embarrassment of being arrogant and later being proven wrong.