imovie preview window looks like filtered

Take a look at this thread and see if anything there helps you:

Video in iMovie is too dark. Movie's produced from iMovie have correct brightness.

-- Rich

Try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Sometimes that cures glitches.

You might also try opening iMovie in a new library by holding down only the Option key while opening, and selecting to open a newly created library in the box that appears. Now see if the preview screen looks filtered.

-- Rich

You might want to consider going back to Mojave. Since installing it on my computer I have had absolutely no issues.

Another thing that you can try is booting up in Safe Mode (see help menu) and, after everything loads, immediately shut down and reboot in normal mode.

-- Rich

Try opening iMovie in a new user account, such as the Guest account on your Mac, or an account that you create. See if the problem persists.

You can use the color correction tool (the one that looks like a palette) to increase the brightness.

You also could try running the first add program from the Disk Utility app found in the Utilities Folder, just to rule out some hardware problem.

Possibly doing another restore of High Sierra might fix it.

Otherwise, this might be something that you can live with, as long as you know that the final product will be O.K. You have tried without success preferences deletion and re-downloading, that usually solve this type of problem. At this point I don't have any more thoughts as to things you can try.

-- Rich

Good point about the color correction. I agree that it probably is not a hardware problem, but what else have you got to try?

There's nothing endemically wrong with Mojave -- it works fine for me and I haven't seen the array of complaints that usually follow immediately after an operating system upgrade. High Sierra has not be known to cause this problem either.

Glitches with iMovie 10.1.9 have been worked out as well, for the most part. So there's nothing wrong with that portion of the software that would be causing this problem, particularly since you already have tried deleting preferences and reinstalling.

As I mentioned, I can't think of any other things that you can try. Fortunately the issue does not affect the final product. You could take your computer into the genius bar at an Apple Store and see if someone with hands on capability can solve it.

-- Rich

Possibly the issue is related to just one project. Have you tried creating a new project and importing new or different media into it?

-- Rich

Sounds like a nightmare, for sure, almost like a virus. Have you run a scan on the iiMac?