Add an adjustment layer, apply curves, and then either draw a mask on the adjustment layer or use another layer as a track matte for the adjustment layer. For example you could create a shape layer the size of the comp with a gradient fill and then either edit the opacity of the gradient with the Edit Gradient feature in the shape layer and use the gradient as an alpha track matte, or edit the gradient color and use the shape layer as a luma matte for the adjustment layer. You can also use the feather tools on a mask to control the boundaries of the effect.
Let us know if you have problems figuring out how to set that up. It's a pretty standard workflow when you use adjustment layers.
Thanks Rick. Can you just expand on how to use the alpha track matte, please? I've now got a shape layer with a gradient, and my adjustment layer beneath it. I don't see "alpha track" in the UI but I'm unfamiliar with AE.
Update: So I construct the gradient on the Shape layer, then Edit > Copy. Select the Adjustment Layer, and Layer > Track Matte? Is that what you suggest?
The track matte controls are in the Modes Panel. The second icon at the bottom left corner of the Timeline will reveal both the switches and modes column in the timeline or you can press F4 to switch back and forth between them. If the second icon is not selected then you should also see a Switches/Modes switch at the bottom of the Switches column.
Here's how I would setup some color correction in this train yard shot to put a faded Kodachrome look on the bottom of the frame without turning the sky yellow.
If you look very carefully between the rail cars you can see the start and ending points for my gradient.
Rick Gerard wrote:
The track matte controls are in the Modes Panel. The second icon at the bottom left corner of the Timeline will reveal both the switches and modes column in the timeline ...
OK, that was the key and I think I've got enough to make progress now. Thanks.