I'm almost certain I read this in the summer of 1980, though it may have been the summer before that. It was a paperback, probably young adult but could be adult, and I think most of the artwork on the cover was in green tones. I also remember it as being a stand-alone novel, not part of a series.
In the story, aliens have ruled the earth for a long time. I want to say the aliens have made the Earth park-like and some characters ride horses, however, it may just be the elite alien's estate that is park-like? In their natural form, the aliens are 6-fingered humanoids, but the ones stationed on Earth go through a transformative process that makes them look almost completely human. The only thing they can't change is their eyes, which are triangular and cinnabar colored.
They've ruled here for long enough that the main character (male) has been raised since childhood (maybe birth?) in their service, and has never left earth. He's extremely loyal and has some special status that causes him to become the student of an elite alien? However, things have reached a point where the aliens decide to abandon Earth. (Human uprising I think?) For some reason, the alien protected by the main character stays behind, and they both end up on the run. I want to say the alien was female, is definitely high ranking, and stays behind because only an alien could operate the transport or the transformation machine or something for the rest? I think the destination they are trying to reach is some special location (on Earth) where technology exists to send her home, and that they succeed.
The only other detail I can remember is that the last alien has a bunch of zoo animals released before they go on the run because the societal collapse that will follow the alien abandonment of Earth means the animals won't be cared for anymore.
Later edit: As noted in the comments this seems to match many of the elements asked about by @Lorendiac "Old SF novel about a white-haired young woman, alien or half-alien, who tutors human kids" In addition to what I've mentioned above, these details also match up:
- The book was on loan to my local library from the local middle or high school library, and so was probably not aimed at full adults. (However, I was already testing at a grade 10 reading level when I was in the 4th grade, so my idea of what constitutes "young adult reading level" may have been seriously skewed!)
- The alien is tutoring 2 humans, 1 male & 1 female, who I remember as being in their late teens or early 20's. In my memory, the male student is the POV character, but it could have been the alien teacher.
- The alien has an unusual, but still human, hair color. Most likely white or silvery-white. After they go on the run she keeps it covered and wears sunglasses to pass as human. At a key point in the story, her hair is seen by an enemy but another character manages to bluff their way out of it. (Or almost does?)