The cotigura is pretty much essential, in my opinion. It is necessary if you don't get the blue, because you'll want to make a slurry on the coticule to polish out the coarse scratches.

You'll also want to make a slurry if you get the blue, because the blue is painfully slow otherwise.

You could make a slurry with the yellow on the blue; since the yellow is harder, it won't wear too much.

But I vote for the cotigura.

Josh