Conventional sartorial wisdom—for whatever that's worth—holds that the combination of black and tan is to be avoided, on the theory that the degree of contrast displeases the eye. Much the same way that we avoid white shirts with black suits (although that's really more about not being mistaken for the wait staff). But to us there's a certain mid-century sophistication about this combination, with the black lending a certain old-world crispness that you can't quite get with navy or brown. The key is simply to not go too light. A British tan or khaki works here; something like stone probably doesn't.