When I use the narrow diamond, I put red arrows on the AMC and DMC when I need to score, otherwise I tend to not use them in this formation as the purpose is to go up the middle. Because of this I dont put arrows on the DL/DR to motivate them: I want them to stay put for the most part.

If your opponent has faster STs, then I would tend to put blue arrows on the DMC to basically give you 5 on defence.

I think in most cases it is safe to put red arrows on the AMC and you can put either blue or red on the DMC depending on your opponent.