If the first odd jacket in any wardrobe is a navy blazer, then the second should surely be something gray. You can make an argument for brown, but brown is less than ideal in the city. Gray on the other hand works for city, suburb and resort wear as well as, in a pinch, after six o'clock.
Have your gray in a three button single breasted, with the lapels rolling to the second button. Patch pockets trump flapped in warmer weather versions, though either is fine the rest of the year.
Gray jackets are the place for neckties with a black ground or, like Cary Grant in To Catch A Thief, with a dark scarf. Pair them with cream, light gray or charcoal trousers.
As Frank and Sammy demonstrate in the original (1960) version of Oceans 11, there ain't nothin' like it.