Rashi on I Kings
The king of Yisroel said to Yehoshofot, I will disguise myself and go into battle. To disguise himself and go [into battle]. I will disguise myself, [i.e.,] I will change my royal garments, so that they will not recognize me, for if they recognize me, they will turn toward me to attack.
Rashi on I Kings
But you wear your own [royal] garments. That you are wearing, for you are not the king of Yisroel, but the king of Yehudah, and they have no conflict with you.