Tones. The military green has an "ash" tone, and goes well with indigo that has a fair bit of sulfur in it, but otherwise, it doesn't work. Likewise, it doesn't work with indigo with a fair bit of red in the dye, like, say APC jeans, at least, unwashed. Navy can work with denim, you just have to find the right tones. It's not even the shade. It's the tone. A cool off primary doesn't work with a warm off primary.