I just need this really basic function on a project over and over. So I slapped this really crude, basic effect together.
Best used with the Histogram scope open in the Viewer. Inspired by Alexis Van Hurkman, born out of need, this uses a Motion "Clone Layer" of the clip, places it on top of the original, defaults to the Add blend mode, then gives you the basic RGB level controls. It uses the Levels filter in Motion 5. By using a copy of the clip above itself, with a Blend Mode, you can create unique looks that a levels filter on on a clip itself would not be able to achieve.
Also I included the Color Balance filter for setting Shadows, Midtones, and Highlights hues. To set the levels traditionally, click the color square (not the arrowhead) to bring up the Apple Color Picker. Go to the color wheel section, click the magnifying glass at the top left, then select the appropriate color you want to set and click on it. All done via a duplicate clip in a Blend Mode for unique effects.