How do I make imovie fade to black and white

To fade to black and white in iMovie, grab the end of the clip and drag the pointer until the section you want to apply the effect to is highlighted in yellow. Then, click on Clip on the top bar and choose Fade To, then Black and White. It won't work if you highlight the entire clip, you must highlight the section where you want the effect.

Use the Fade through Black transition at the end of your movie to fade to black.

Use the Fade Through White transition at the end of your movie to fade to white.

Or use these transitions between any two clips.