Short direct answer to the problem:
For at least two movies I get pretty good results by turning the brightness down to near -50% or by moving the lower level control up to the 35% region, as PadraigQ suggests. Not sure which is better. Both introduced some blue in green bushes in the shade (dark, as the foreground is in the sun). The Hard Light helped, but I don't use it. It degrades the image and turns dark grey tones to black.
More (a lot more, I'm afraid!):
Yeah, Apple recommends to update iMovie. What a surprise. (Sorry, Apple. :-)
But iMovie 10 is troublesome. Here's why:
First of all, iMovie 10 is largely dumbed down. Even iMovie 9.0.9 is dumbed down compared to 9.0.4 for at least one thing: Where did the red, green, and blue gain sliders go?
This was from a couple of years ago, so I'm a little fuzzy on it, but I know I quickly found that iMovie 10 was not for me. The only thing I use it for is as a stepping stone to transfer projects from iMovie 9 into FCP. I'm told that you can't go directly from 9 to FCP.
I was unable to import/convert my 9.0.4 library in one fell swoop. It would crap out early in; I forget exactly how. Lovely. I eventually found I could import a few at a time by moving some of the files to a side directory, though I can't find it now. Maybe the latest version will work better.
Another problem going to iMovie 10 was losing the video settings, as the controls between 9 and 10 are incompatible. This is a good thing? iMovie 10 just gives you saturation and tint, whereas 9.0.4 has saturation, and red, green, and blue sliders. Instead of exposure (never useful), levels, and contrast you get some strange controls that kind of combine them. Could be useful. Oh, iMovie 10 also has the skin-tone and match-color features, which also could be useful, but these features don't make up for all the problems.
Yet another problem with iMovie 10 is the aspect ratio. My movies are Super 8 transfers rendered as 1440x1080. They come up as 16:9 instead of 4:3. Has something with ignoring the value of the anamorphic bit. There was no way I could find to fix it. In FCP I was _barely_ able to fix it. Barely. I tried all sorts of things. Then I found one that seemed to do the trick, but later found that it messed up the cropping. I found another that actually seems to work properly. QuickTime 7 Pro, OTOH, gets the aspect ratio correct, right from the get-go. Long live QT 7 Pro!
Still yet another problem: I opened iMovie 10 today for the first time in a long time and I can't change what's in the timeline. I can't open a project! (v 10.1.7) This is after _having to abort_ the library conversion thing (after half an hour!). That must be it (still further yet another problem). I can, however, get a clip in the browser and viewer. Looks like it's time for a support call or perhaps a new post.
There's more with 9 vs 10, but this post is already quite long. And there's only so much time in the day!
Going back to 9.0.9, I found today that I can't do any video adjustments for a clip that has a fade in and a fade out! This is an upgrade?
iMovie 10 is not an upgrade from 9; it is a different app altogether.
Note to Apple: If you're going to break things, please just retire the old app (and continue to support it!) and make the new one a totally separate app. Or give us a hidden "advanced controls" panel that makes things backwards-compatible. And can you please give us a checkbox for making 1440x1080 4:3? You do for other formats. Thank you! (^_^)