From the Furminator web site at:
http://www.furminator.com/

Q: Which size tool is best for my pet?
A: All of the tools have the same edge, only the width is different.

* Small Blue Tool (1.75" wide blade) for small to medium sized dogs, cats, rabbits and ferrets

* Purple Cat Tool (1.75" wide blade) for small to medium sized cats

* Medium Yellow Tool (2.65" wide blade) for medium to large sized dogs

* Large Yellow Tool (4: wide blade) for large dogs

* Hunter Green Equine Tool (5" wide blade) for horses and other livestock

There's no distinction made between the "Blue" and the "Purple" tools -
The Furminator site gives the same 1.75" blade dimension for both.

Q: Do I ever need to sharpen or replace the deShedding edge?
A: The average consumer should not have to replace the deShedding edge for the lifetime of the pet.
We always advise to keep the protective cover on the deShedding edge at all times when not in use.
The small blue and purple cat deShedding tools do not have replaceable edges.
The medium, large and equine deShedding tools do have replaceable edges.

So, what SIZE are you folks using??