You can adjust the luminance curves of the front and back mattes of your composite by selecting the layer from the 2D Compositor tool.

The 2D Compositor tool UI is divided into four areas:

To adjust the matte's luminance curves:

1. In the Layer Browser, select the composite layer to adjust. You can mute or solo the layer.
2. Use the Layer Tools to navigate through the 2D Compositor's layers.
3. Select mathematical blending operations from a context menu.
4. Once a layer is selected from the Layer Browser, click a curve to select it. Adjust the luminance curve by clicking a point to display its tangent handle and drag the handle to adjust the curve. Using hotkeys, you can add and delete control points, break or unify tangent handles—see Animation Editor.