Ah, yes, the 'don't iron the vilene' rule. Good rule.
Personally, with string quilts, I think the element that pulls them all together is the center strip being consistent.
You might want to consider doing all the blocks with a neutral center, one all blue side and one multi. I think this would give you several different layouts. I have seen scrap quilts in all batiks, and batik/quilting cotton mixes. If you don't like the mixed look, then go with all one or the other. As far as lights and darks, another way to go would be darks on one side and lights on the other. Not a lot of help!