Attached Link: Best Practices for Masking

Thanks for the tip Touchwood, it put me on the right path.  I did a search for masking in the compositor and found the attached link which will prove helpful.  It uses the difference, colour and distance nodes in the compositor to mask the selected colour.  

In my situation this method has its limitations as it is removing the colour post render. This means that if any of the objects being rendered in the mirror have the same colour as the world colour then those colours get removed as well.  If I change the world colour to something that is different to the rendered objects in order to create a clean mask it leaves an incorrect colour tint on the rendered reflected objects. 

Again thanks for the help.

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