Unpremultiplying and Premultiplying

When compositing for CG you will get various passes that you will have to rebuild back together and then add some things such as color corrections. That is where the issue comes in. Color correcting after premultiplying your rgba layer can change the color of your RGB falloff, and it will no longer match the falloff of your alpha, which means you may get some undesirable edges in your comp. The Premult node does not effect your alpha channel so what you would want to do is first use the Unpremult node on any of your passes that has an alpha channel, do all your color corrections, and then premult.