I think the shirts are Bugatchi Uomo. The coordinating color fabric with a different design is one of the trademarks.
According to Eric Stonestreet, who plays Cam, all the shirts are custom made:
There’s a custom shirt maker in L.A. that makes my shirts, Anto of Beverly Hills, and they have made shirts for every famous person you can think of. They made shirts for Frank Sinatra, Ronald Reagan, Dean Martin – you name them, they’ve made their shirts since the ’30s and ’40s. It is such a treat every day to come into my trailer and see what the shirt looks like. I don’t get to pick the fabrics or anything like that. I just help pick the style and then they just go crazy with what I wear.
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them