gn_wendy - 7:22 am on Apr 16, 2010 (gmt 0)
i'm with what tedster is saying.
however - to respond to the specific question:
in the case of a COLOR drill down or a results per page change, should I use the canonical tag
Just to make sure i am getting this right you have some sort of "search filter". I am going to say you sell shoes (because i need an example). If I search your page for tennis shoes i will probably land on example.com/tennis-shoes/
If I understand you correctly you send me to a new page if I only want to see red tennis shoes. Here i would not use the canonical tag. I would however probably set it to "noindex" depending on how many backlinks you have and how well you are indexed. You could always remove the noindex tag for a small subseciton and test what the effect is.
In the next example you gave us where you have /show-all and /show24perpage etc. I would use the canonical tag AND the noindex tag, because that will definately trigger a duplicate content issue. You are serving identical information for:
parts of /tennis-shoes/
and all of /tennis-shoes/red AND /tennis-shoes/red/show-all AND all other show# pages.