An overcoat's function is to keep your warm and dry in very bad winter weather. It must therefore offer enough thickness and coverage to protect your whole body against heavy rain and hail, and to keep you warm and well-insulated as well.
This rules out knee-length coats or shorter - they leave your lower legs exposed in heavy weather. Even if you have an umbrella, sometimes the wind will make it unusable, or bring the rain in at a sharp ankle; and umbrellas provide no protection against splashes from passing vehicles. If you are wearing a suit, then your trousers could be soaked through in bad weather if you have a knee-length coat.
Double-breasted overcoats clearly provide more insulation than single-breasted, due to the extra layer over the chest.
As for colour, overcoats are worn in bad winter weather, when there is a lot of mud, dirt, and so on. So, lighter colours will look grimy very quickly. Overcoats are also typically worn over suits, to protect them from damage. Even if you wear suits infrequently, you want an overcoat that can go with a suit and tie, just to be versatile. Darker colours go better with business suits. Finally, dark colours suit winter better than light colours. So, I would say that navy is the best colour, with charcoal grey coming second. Camel looks great but shows dirt quicker, and is more casual and is more of an autumn/spring colour - I think camel is better for a raincoat or trench coat rather than a heavy-duty overcoat.
So, in my opinion, a navy calf-length double-breasted is the best combination for a first overcoat. Any thoughts?
