I want to add a black and white as well as a film grain effect to the same video in iMovie 10. I can't see any way to do this as applying one effect removes the other. Does anyone know how to do this? Thanks.
Not in iMovie 10.0.5, unless you open up a separate project, drag the clip in there, apply the 1st effect, export it, then drag the exported clip it into your main project or 2nd project (if 3 effects or more effects are needed), then add a second (or final effect).
You can try applying one effect first and then by exporting and re-importing you can apply the second effect.
Click on the video clip, then go to the color palette and bring it all the way down, it will make the clip black and white. Then go in to the effects and add the aged film.