The iMovie video effect "Exposure Adjust" (found in Quartz Composer) makes the entire video clip lighter or darker.
iMovie Video Effects - Exposure Adjust
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.
EV: Move this slider toward Max to increase the exposure to the clip (making it brighter) or toward Min to decrease the exposure (making the clip darker).