The iMovie video effect "Sepia Tone" gives video clips the brown and white look of old photographs.
iMovie Video Effects - Sepia Tone
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.
Intensity: Use this slider to adjust the intensity of the sepia tone. Max makes the clip entirely sepia; moving the slider toward Min just gives it a slight sepia tinge.