Wiki: "Harper (released in the UK as The Moving Target) is a 1966 American Technicolor American neo-noir. The reference is to Silver, Alain; Ward, Elizabeth; eds. (1992). Film Noir: An Encyclopedic Reference to the American Style (3rd ed.).
https://rateyourmusic.com/film/harper/ classes it as neo-noir. That site's first review says "Belongs firmly in the pantheon of The Big Sleep, Chinatown, and The Big Lebowski."
Review #2 says "Although we can now place it at the beginning of the neo-noir revival that really got going around 1970, it is very much a 1960s film, not a self-conscious noir - and it is shot by Conrad Hall in the brightest Californian sun light."
It doesn't drop the 60s trappings, it cannot, anymore than The Long Goodbye can. The betrayal by friends is a similarity. So is the trip through California types. The part about brightness is dead wrong. Watch the whole film and you'll probably be surprised at how many night scenes and dark interiors there are, maybe 80% of the film.
Here's an observation from yet another site: "The color in Harper is not garish at all, which is sometimes true of other Technicolor films from the 1960s; rather, it is muted and subdued. At times, color fills the screen, making some shots feel cramped almost to the point of suffocation." This use of color definitely looks forward to its use in the neo-noir flood. (http://makeminefilmnoir.blogspot.com/2019/06/harper-1966.html)
I could go on, as with http://intercut.net/episode-1-of-the-intercut-1970s-neo-noir-series/. This groups Harper, Point Blank and Klute together as neo-noir films of the 70s. They discuss 13 such films.