I am very new to After Effects. I have recently discovered how to use a mask to separate colour and leave just one colour. This works well and I even know how to use rotoscoping to animate this effect so that the same area stays with the single colour. The problem I have is that I need to change the shape of the mask as when you advance the playhead over the timeline the object being masked moves, leaving the mask in the same place. What I need is to be able to change the shape of the mask as the object moves.
Hope I'm explaining myself properly so that you can understand what I am saying. Thanks for your time.
I'm using After Effects CS6.
Select the layer with the mask in the timeline. Press the M key. See the little stop watch symbol? Click there. Now you can animate. You could also use the keyboard shortcut Alt/Option + M to set a mask keyframe and reveal the Mask property in the timeline. Using the keyboard shortcut will set keyframes for or remove keyframes for every mask on that layer.
You could also simply turn on Auto Keyframe Properties at the top of the timeline. It looks like a little stop watch and it's just to the left of the Graph Editor Switch. It's red when on. I never activate this switch but some people like it.