Composite node group. Just plug-in render layers, adjust what You need, and everything will be automatically composited. This is a node to connect without any other things render passes, also it has some adjustments. Amount of given render pass, hue, saturation, value and off course mask for that adjustments.
It works with every render pass in Blender/Cycles.
That group is inside the Blend file. To use it another project, just append it from that file.