The iMovie video effect "Adjust Colors" lets you adjust many aspects of the color of your video clips, including the hue, vividness, and brightness.
iMovie Video Effects - Adjust Colors
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.
Hue Shift: To adjust the hue of your clip, move the slider toward Left or Right. This will effect different clips differently; play with it to find a hue adjustment that you like.
Color:To increase the color saturation of a clip, move the slider bar toward Vivid. Moving the slider toward B&W will decrease the color saturation in the clip.
Lightness:Move this slider bar to make the colors in the video clip brighter or darker.