(Dark) pink shirts were originally part of the officer’s uniform in the Army Air Corps in World War II.
https://www.worthpoint.com/worthopedia/original-ww2-uniform-pink-dress-shirt-485421314
They were worn as a variant of the standard Army Pinks & Greens uniform which had a khaki shirt.
Brooks Brothers later popularized the wearing of pink dress shirts.
And everyone wearing a pink shirt, when asked will tell you it’s salmon!