Without knowing the variety, or some close up shots, I can't be 100% certain, but from this distance, it looks like they are just turning brown as part of the natural growth pattern. I'm also in zone 5 and just harvest 650 heads of my early harvest garlics. Your look a bit smaller than mine, but the leaf color patters are similar. Did you plant in October or this spring?

I would suggest pulling one or two and examining the head. You should see plenty of wrapping material still present. Look at the stalk. Is it still strong and vibrant. If you leave garlic in too long, the cloves lose the wrappings, start to separate and the stalk begins to rot. Then, when you try to pull the head, the stalk breaks and you have to dig up by hand. If the wrappings are gone it will be good only for seed stock as it will not cure and keep at all.