Simple Editing of RGB Channels On Mac
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.)