Hugh Dingle and I stopped at a small patch of flowering Eucalyptus on the corner of Pleasant's Valley and Pleasant Hill Ranch Way where there were 5-6+ Anna's Hummingbirds and one other, which we saw briefly, but despite looking for another ~15-20 minutes, but it never came back.
We saw it fly in and perch, very briefly, at an angle to us, about 3/4 of the way up a Euc (so probably 20-25' from our vantage point). It was obviously white underneath, in stark contrast the nearby Anna's, this is what made us both pick up on it immediately. The chin was dark, though I am not sure whether it was reflective. I did not get a good view of the head, but it had some white on the sides of the neck or nape (unfortunately, as we were below and at an angle, I am not sure where this was, exactly). The white below was extensive, nearly the entire belly and chest. Hugh thought it was shinier than the surrounding Anna's, though I did not notice this. Unfortunately, we did not get a view of the tail, nor any vocalizations. While I saw the bill, I do not recall any characteristics. Neither of us saw buffy flanks, either.
We watched it for probably about 8-10 seconds, so it was not a fleeting glance, but neither did we have time to carefully study it.
It stuck out from the other Anna's (including females) around, so I don't think it was an abnormally light female, but I also can't rule that out.
The only other bird of note was a Loggerhead Shrike near the bottom of Mix Canyon. The top of the ridge was completely dead this morning - barely anything moving, despite being warm and sunny.