A man once entered a Beit Midrash and inquired of a student as to the whereabouts of his Rabbi. The student replied, "He went to a Berit Mila with his colleagues." The visitor then asked the student to identify these colleagues, and it turned out that these "colleagues" were actually the Rabbis of that student's Rabbi. The student had referred to them as his Rabbi's "colleagues" out of respect for his Rabbi, but the visitor berated the student for speaking disrespectfully about his Rabbi's Rabbis.
The Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1833-1909), in his work "Torah Li'Shma," addresses the question of whether the student had indeed infringed upon the Rabbis' honor by speaking of them as his Rabbi's "colleagues." He proves that this is permissible from an incident recorded in Masechet Beitza (5). Rabbi Eliezer mentioned a certain Halacha to his students, and they responded, "Your colleagues have already repealed this Halacha." The Gemara clarifies that it was Rabban Yochanan Ben Zakai who had overruled the Halacha in question. We know from other sources that Rabban Yochanan Ben Zakai was Rabbi Eliezer's mentor, and yet Rabbi Eliezer's students referred to him as Rabbi Eliezer's "colleague" out of respect for their Rabbi.
Thus, the Ben Ish Chai concludes, it is permissible for a student to refer to his Rabbi's Rabbi as his "colleague" as an expression of respect.