I asked this same question and was told by a Zodiac service tech that we replace the gray part# with the same gray part. That's what I did a couple weeks back and all is well. I was told that we are stuck with gray until we end up purchasing the complete motor unit at $300 from Zodiac. I had a pool guy come replace my motor unit 2 years back just like you and he installed the motor with the gray sensor.
The funniest part to me was that he moved to Arizona. Maybe we had the same guy? Haha