Simple Editing of RGB Channels On Mac In older versions of iMovie you could edit the RGB channels of a clip independently. In the current iMovie 10 you can't. It doesn't seem to be a widespread feature, tried Shotcut and Video Pad so far as well as QuickTime Player and VLC.

Do you know of a free or low priced Mac app that can do this? Final Cut and After Effects are overkill for just this quite basic function.
Apple's Motion has a wide variety of useful effects for this sort of thing. It's not clear which iMovie filter you were using (a built-in one, or a 3rd party one?), but Motion offers a Channel Mixer filter, and includes a Levels filter which can work on individual channels. (You can see the docs for Color Correction Filters here.) In addition, Motion also supports 3rd party effects, and there are a ton of them that could do what you need. (It's hard to give exact details without more info from you about what you need, though.)