Edges and colour

• Match Color grades the layer to the mean and contrast of the render around it, or of the selection, with a Color Balance node in the layer's own nodes.
• Choke shrinks the edge to drop halos.
• Light Wrap spills the blurred background over the layer's inner edge.
• Grain adds noise that changes every frame, only where the layer is.

Layer nodes

For anything more, give the layer its own nodes. Add Nodes gives it a node group (Image in, Image out) applied to its pixels before its mask and placement, and opens it in the Compositor editor. Edit in Compositor reopens it.

• Several layers can share one group.
• CS Layers is rebuilt on every change but only references these groups, so the nodes you add are never touched.