Animating Color to Monochrome

Make your own Pleasantville effect by using the Leave Color Effect.

Select a footage layer on choose Effect>Stylize>Leave Color.
Use the Eyedropper to choose the color you want to keep, and change the Amount to Decolor to 100 percent.
Go through each frame to check to see if you have achieved the effect you want.
Use the Eyedropper to select colors at key points to fix discrepancies.

A great way to use this effect is to use it in conjunction with Time Remapping. Apply Time Remapping to a layer and freeze the action for a few seconds. Apply the Leave Color Effect to focus attention on a part of the layer.

You can effectively turn this effect off by increasing the tolerance to 100 percent. Select the keyframes and choose Animation>Toggle Hold Keyframe to turn leave color on for the few seconds the video is frozen.