Anna O said it very well.
I've done several double irish chains (all from Quilt in a Day) and I love them. My suggestion would be arrange by value. Darker for center of chain (in the case of 6 maybe choose 2), lighter for the next (again 2) and then the lightest (again 2) as the area between the chains. If you sort this ahead of time you can mix the like value fabrics as you wish.