Im new to after effects cs4. I made a time lapse video of the sky. However, the sky is to bright and you can't really see the clouds very well. I would like to change the 'levels' of the sky, but not the buildings at the bottom of the video. So only half of the video will have colour correction on it. How do i do this???? Thanks!
The gist is simple:
The devil is in the details. For example, if you need to animate the mask, you may have some rotoscoping to do.
Here's a video that shows how to do this in a case where the item to be color-corrected is moving:
This is actually a very basic compositing question, so I'm going to strongly recommend that you begin at the beginning and work your way through the getting-started learning materials.