Use the Brightness & Contrast video effect to adjust the look of the colors in your iMovie video project.
iMovie Video Effects - Brightness & Contrast
Effect In, Effect Out:To apply the effect to only part of the video clip, move the sliders to the places/times when you want the effect to begin and end. Leave them both set to 0:00 to apply the effect to the entire clip.
Brightness: Move the slider toward Bright to increase the exposure of the video clip, or toward Dark to decrease the exposure.
Contrast: Move the slider toward Low to decrease the contrast between light and dark areas of the video clip, or toward High to increase the contrast.