Graduated Mask in iMovie

I do not know of any way in iMovie to do the gradient effect that you describe. There is no gradient feature in iMovie. You would need to use an external app.

One possibility would be to create a blue gradient rectangle in the free Keynote app, and then export it as an image to your desktop, and from there into iMovie. Put it in into the main timeline and then place your video above it and use the Blue/Green screen effect. That would make the sky more vibrant. You could then export the project to your desktop and import it back into a new iMovie project. Then add a green gradient to the bottom and Green Screen it to make the landscape more vibrant.

-- Rich

Here's an Apple help bulletin that explains how to use the Green/Blue Screen effect:

https://support.apple.com/en-us/HT210891

The first part of the help bulletin describes how to use Green Screen with iPad and iPhone. Skip over that because it doesn't apply to iMovie for Mac. Instead scroll down to midway for an explanation pertaining to the Mac.

Take a look at the first screen shot that I posted. I put the gradient background made from KeyNote into the main (bottom) timeline. Then I put the video to which I want to add a more vibrant sky into the timeline above the gradient clip. Then select the video clip in the upper timeline, and click on the overlap tool (the one that looks like overlapping rectangles) in the tool bar in the upper right of the screen. When you click on the overlay icon a little selection box will appear that will let you select Green/Blue Screen. Once you have added the Green/Blue Screen effect it will remove the blue from the video clip so that the blue from the gradient clip below will show through instead. To apply your effect, click on the little blue reset button at the upper. right of your screen.

You can drag out the right edge of the gradient clip to equal the full duration of your video.

-- Rich

Looks like you are doing a very good job.

Maybe you could create a blended gradient and green screen it:

Or use a photo editor to soften the edge:

You might try taking a screen shot of one or both of the above and see if it works, as a test.

Also, you could simply use one of the iMovie filters (click on the 3 overlapping color wheels in the tool bar at the top right of your screen) to get the effect your want. For instance:

You could change this . . .

. . . into this . . .

-- Rich